Sleep Architecture And Cortisol Deep Sleep

Last updated: October 4, 2026 - Reviewed by Verdant Wellness Editorial Team

Table of Contents

1. What Is Sleep Architecture?
2. Cortisol's Role In The Sleep-Wake Cycle
3. Deep Sleep And Cortisol: The Core Relationship
4. Slow Wave Sleep And Cortisol Suppression
5. REM Sleep And Stress Hormones
6. NREM Cortisol Dynamics Across The Night
7. The Deep Sleep HPA Axis Connection
8. Pre-Sleep Cortisol And Sleep Efficiency: What A 2024 Study Found
9. How Stress Changes Sleep Architecture: 2026 Research
10. Deep Sleep Cortisol Clearance: What It Means For Recovery
11. Sleep Stages Cortisol Research: What EEG Studies Reveal
12. Does Insomnia Cause High Cortisol Or Does Cortisol Cause Insomnia?
13. Which Sleep Stage Is Most Vulnerable To Stress?
14. Interventions That Improve Sleep Architecture And Support healthy cortisol
15. Biomarkers For Tracking Sleep Quality Beyond Subjective Reports
16. Key Takeaways And Practical Summary

Introduction

If you have ever woken at 3 a.m. with your heart pounding and your mind racing, you have experienced the intersection of sleep architecture and cortisol deep sleep biology firsthand. That sudden, unwelcome alertness is not random. It is, in many cases, a cortisol spike pulling you out of restorative sleep at precisely the wrong moment.

The relationship between sleep staging and stress hormone biology is one of the most actively researched areas in modern sleep science. In 2024 alone, a landmark longitudinal study published in the Oxford Academic journal SLEEP tracked 15 consecutive nights of EEG-assessed sleep alongside 14 days of cortisol sampling to show — with striking granularity — that higher pre-sleep cortisol predicted shorter total sleep time, worse sleep efficiency, and longer sleep onset latency on a day-to-day basis within individual participants. And in 2026, new research confirmed that sustained daily stress tilts sleep architecture away from deep restorative N3 sleep and toward lighter, more fragmented REM-dominated patterns.

This post synthesises the most current research on sleep architecture cortisol interactions, explains why deep sleep functions as a biological cortisol-clearing mechanism, examines what happens to the hypothalamic-pituitary-adrenal (HPA) axis during different sleep stages, and offers evidence-informed strategies for protecting the slow-wave sleep your body depends on for hormonal restoration.

Whether you are a clinician, a researcher, a biohacker, or simply someone who has been sleeping badly and wants to understand why, you will find the full mechanistic picture here.

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1. What Is Sleep Architecture?

Sleep architecture refers to the structural organisation of sleep across a full night — the sequence, duration, and proportion of each sleep stage as they cycle from sleep onset through to morning waking. Rather than a uniform, passive state, sleep is a highly organised biological process involving four distinct stages that repeat in cycles of approximately 90 minutes.

The Four Sleep Stages

Stage N1 (Light NREM Sleep) N1 is the brief transitional phase between wakefulness and sleep. It typically lasts only one to seven minutes per cycle. Brain activity slows from the waking alpha and beta rhythms to slower theta waves. Muscle tone decreases, and the sleeper is easily roused. From a hormonal perspective, this stage offers little in the way of cortisol suppression or HPA axis downregulation.

Stage N2 (Intermediate NREM Sleep) N2 makes up the largest proportion of total sleep time in healthy adults — roughly 45 to 55 percent. It is characterised by sleep spindles (short bursts of 12–16 Hz activity) and K-complexes. Body temperature drops, heart rate slows, and the brain begins more active memory consolidation processes. Cortisol levels continue to decline during this stage as the early part of the night progresses.

Stage N3 (Slow-Wave Sleep / Deep Sleep) N3 is often called slow-wave sleep (SWS) or deep sleep and is defined by high-amplitude, low-frequency delta waves (0.5–4 Hz) comprising at least 20 percent of the epoch. This is the most restorative sleep stage. Growth hormone secretion peaks during N3, protein synthesis accelerates, immune function is bolstered, and — crucially for this discussion — cortisol reaches its nadir. Deep sleep is concentrated heavily in the first half of the night.

Stage R (REM Sleep) Rapid eye movement (REM) sleep is physiologically distinct from NREM sleep. The brain is highly active, producing patterns similar to wakefulness. Skeletal muscles are temporarily paralysed (atonia). REM is associated with vivid dreaming, emotional memory processing, and — importantly — rising cortisol levels, particularly in the second half of the night when REM cycles dominate.

Normal Sleep Architecture In Adults

A typical healthy night involves four to six 90-minute cycles. In the first half of the night, N3 deep sleep dominates. In the second half, REM cycles lengthen. The proportion of N3 sleep decreases with age — a fact with significant implications for cortisol regulation in older populations.

Understanding this architecture is essential because stress, cortisol dysregulation, and HPA axis overactivation do not affect all sleep stages equally. Research is increasingly clear that N3 deep sleep and REM sleep are affected in opposite and sometimes paradoxical ways when cortisol and stress enter the picture.

2. Cortisol's Role In The Sleep-Wake Cycle

Cortisol is a glucocorticoid hormone produced by the adrenal cortex under the direction of the hypothalamic-pituitary-adrenal (HPA) axis. Its primary function is to mobilise energy and coordinate the body's response to psychological and physiological stress. But cortisol is not simply an "emergency" hormone — it is a fundamental regulator of the sleep-wake cycle.

The Cortisol Diurnal Rhythm

Under normal circumstances, cortisol follows a well-defined 24-hour rhythm that is closely synchronised with the circadian clock:

• Cortisol nadir: Occurs in the first half of the night, roughly between midnight and 2–3 a.m., coinciding with peak slow-wave sleep.
• Cortisol awakening response (CAR): Cortisol surges sharply in the 30 to 45 minutes after waking — rising by 50 to 160 percent — to mobilise glucose and prepare the body for the demands of the day.
• Daytime decline: Cortisol levels decline gradually throughout the afternoon and evening.
• Evening suppression: By bedtime, cortisol should be at near-nadir levels, facilitating sleep onset and early deep sleep.

This rhythm is maintained by the suprachiasmatic nucleus (SCN) of the hypothalamus, which synchronises HPA axis activity with the light-dark cycle. When this rhythm is disrupted — by shift work, chronic stress, light exposure at night, or poor sleep — the downstream hormonal consequences include impaired deep sleep, elevated pre-sleep cortisol, and compromised next-day stress resilience.

Cortisol And Sleep-Related Hormone Interactions

Cortisol does not operate in isolation during the sleep period. A 2004 study published in Nature (Held et al.) examined sleep-related hormonal secretion patterns and found that sleep-related secretion of cortisol and ghrelin (an appetite-stimulating hormone) decreased throughout the night, while leptin (a satiety hormone) and melatonin increased. This hormonal orchestration across the sleep period reflects a deeply integrated biological programme in which cortisol suppression during deep sleep is part of a broader anabolic, restorative hormonal milieu.

Disrupting cortisol suppression — whether through psychological stress, poor sleep hygiene, HPA axis dysregulation, or clinical disorders — therefore cascades into disrupted leptin, ghrelin, melatonin, and growth hormone dynamics, with broad implications for metabolic health, appetite regulation, immune function, and emotional well-being.

3. Deep Sleep And Cortisol: The Core Relationship

The relationship between deep sleep and cortisol is bidirectional, reciprocal, and biologically fundamental. Deep sleep suppresses cortisol. And high cortisol suppresses deep sleep. Understanding which direction the causality flows — and when — is one of the central questions in sleep-stress research.

How Deep Sleep Suppresses Cortisol

During N3 slow-wave sleep, the HPA axis enters a state of relative suppression. Several mechanisms contribute to this:

1. Increased GABA ergic activity: Deep sleep is associated with upregulation of gamma-aminobutyric acid (GABA), the brain's primary inhibitory neurotransmitter. GABA inhibits CRH (corticotropin-releasing hormone) neurons in the hypothalamus, reducing the drive to ACTH secretion and subsequently to cortisol release.

1. Growth hormone surge: The pronounced growth hormone (GH) pulse that occurs during early N3 sleep directly inhibits HPA axis activity. GH and cortisol have largely opposing catabolic-anabolic relationships, and GH's dominance during deep sleep helps maintain cortisol at its nadir.

1. Autonomic shift to parasympathetic dominance: Deep sleep is characterised by reduced sympathetic nervous system (SNS) activity and increased parasympathetic tone. Since the SNS provides significant drive to the adrenal medulla (for adrenaline/epinephrine) and modulates HPA axis activity, parasympathetic dominance during N3 is inherently cortisol-suppressive.

1. Reduced amygdala reactivity: Neuroimaging studies show that amygdala activity — a key driver of stress-induced cortisol release — is significantly reduced during deep sleep, limiting the emotional arousal signals that would otherwise activate the HPA axis.

How High Cortisol Disrupts Deep Sleep

The inhibitory relationship runs in both directions. When pre-sleep cortisol is elevated — whether from a stressful day, evening exercise, late caffeine consumption, artificial light exposure, or chronic psychological stress — it directly interferes with the initiation and maintenance of N3 slow-wave sleep through multiple pathways:

• CRH-driven arousal: Elevated cortisol is upstream of and correlated with elevated CRH, which has direct arousal-promoting effects in the brain, increasing wakefulness and reducing deep sleep propensity.
• Hyperarousal of the HPA axis: Chronic HPA axis activation maintains elevated baseline cortisol, which flattens the normal nocturnal cortisol nadir and reduces the hormonal conditions that favour deep sleep architecture.
• Increased sleep fragmentation: High cortisol reduces sleep depth and increases the probability of brief awakenings, preventing the uninterrupted slow-wave sleep periods needed for full restorative benefit.
• Delayed sleep onset: As the 2024 Oxford Academic study demonstrated, higher pre-sleep cortisol was associated with significantly longer sleep onset latency — the time it takes to fall asleep — within individual participants on high-cortisol nights compared to their own low-cortisol nights.

This reciprocal suppression creates the potential for a vicious cycle: high cortisol reduces N3 deep sleep → reduced N3 deep sleep impairs cortisol clearance → elevated morning and next-evening cortisol further disrupts the following night's deep sleep → and so on. Breaking this cycle is the central goal of most sleep-stress interventions.

4. Slow Wave Sleep And Cortisol Suppression

Slow wave sleep and cortisol have perhaps the most clearly defined relationship of any sleep-hormone pairing. The electrophysiological signature of N3 — large, synchronised delta waves sweeping across the cortex — appears to be not merely correlated with cortisol suppression but may actively drive it.

Delta Power And HPA Inhibition

Sleep researchers measure slow-wave activity (SWA) as the power density of EEG signals in the 0.5–4 Hz range. Higher delta power = deeper, more restorative slow-wave sleep. Studies have found that delta power is inversely correlated with cortisol levels during the sleep period — the higher the slow-wave activity, the lower the cortisol.

This relationship has been demonstrated in studies using polysomnography (PSG) alongside blood sampling via intravenous cannula or repeated salivary collections. The suppression of cortisol during peak delta activity appears to be more than coincidental timing — it reflects active HPA axis inhibition driven by the neurobiological state of deep sleep.

Clinical Evidence: Mirtazapine, SWS, And Cortisol

A compelling demonstration of the slow wave sleep cortisol relationship comes from the clinical pharmacology literature. A 2004 study published in Nature examined patients with depression — a condition characterised by both HPA axis hyperactivation and suppressed deep sleep — treated with mirtazapine, an antidepressant with known sleep-promoting properties.

The study found that after four weeks of mirtazapine treatment:

• Sleep continuity improved rapidly (within the first week)
• By week four, slow-wave sleep (SWS) had significantly increased
• The combined dexamethasone-CRH (DEX-CRH) test — the gold-standard measure of HPA axis reactivity — showed a distinct reduction in ACTH and cortisol response

This temporal sequence is telling. The improvements in HPA axis function — measured by the DEX-CRH test — followed and coincided with the increases in slow-wave sleep. While mirtazapine has multiple pharmacological actions that could independently affect the HPA axis, the parallel improvement in SWS and cortisol normalisation is consistent with the hypothesis that restorative slow-wave sleep is a mechanism through which HPA axis tone is regulated.

Sleep Deprivation And Lost SWS

Conversely, when slow-wave sleep is selectively suppressed — either through stress, sleep fragmentation, substance use, or experimental methods — cortisol dysregulation follows. Studies using selective SWS suppression (where experimenters play tones to disrupt deep sleep without fully waking participants) have shown that even partial SWS deprivation elevates morning cortisol and impairs next-day stress reactivity.

This makes biological sense: if slow wave sleep and cortisol suppression are mechanistically linked, then reducing the former should predictably impair the latter.

5. REM Sleep And Stress Hormones

While deep sleep is associated with cortisol's nadir and active HPA suppression, REM sleep stress hormones tell a different and more complex story. REM sleep, which predominates in the second half of the night, is associated with rising — not falling — cortisol levels, along with a distinctive pattern of autonomic and hormonal activation.

REM Sleep Physiology And Cortisol

During REM sleep, despite skeletal muscle atonia, the brain is highly active and metabolically demanding. Autonomic activity is variable and often elevated, with brief bursts of sympathetic activation corresponding to the phasic (burst) components of REM. Cortisol levels, which reach their nadir in early N3 sleep, begin rising during the later NREM stages and continue rising through REM cycles in the second half of the night.

By the final REM cycle before morning waking, cortisol has typically risen to moderate levels in preparation for the cortisol awakening response (CAR). This pre-waking cortisol rise is thought to help transition the body from sleep to wakefulness and represents a normal, expected feature of healthy REM sleep architecture.

However, the relationship becomes pathological when cortisol is abnormally elevated during REM sleep, or when stress increases REM sleep proportion at the expense of N3 deep sleep — as recent research confirms.

2026 Research: Stress Increases REM, Decreases Deep Sleep

A 2026 study published in ScienceDirect titled The Impact of Stress on Sleep Architecture: A Seven-Day Study provides perhaps the most ecologically valid recent evidence on this question. Across a seven-day study period, researchers found that higher daily stress was associated with increased REM sleep percentage and decreased deep (N3) sleep percentage.

This pattern — more REM, less N3 — is consistent with a stress-driven shift in sleep architecture that prioritises lighter, emotionally active sleep at the expense of the physically and hormonally restorative deep sleep that the body most needs when under duress. The cruel irony of stress-related sleep disruption is that stress removes precisely the sleep stages most capable of restoring the biological systems that stress depletes.

REM Sleep, Emotional Processing, And Cortisol

REM sleep has a well-documented role in emotional memory processing — sometimes described as "overnight therapy." During healthy REM sleep, the brain replays emotionally charged memories in a neurochemical context low in noradrenaline, which is thought to allow emotional desensitisation and reprocessing. When REM sleep is excessive or fragmented — as occurs under stress — this processing becomes less efficient and may actually reinforce rather than dampen emotional reactivity and HPA axis sensitisation.

The relationship between REM sleep stress hormones is therefore not simply that REM sleep is "bad" from a cortisol perspective. In healthy proportions and timing, REM serves important regulatory functions. The problem arises when stress-driven cortisol dysregulation reshapes sleep architecture to create excessive, poorly consolidated REM at the expense of deep slow-wave sleep — producing a hormonal and emotional cycle that is self-perpetuating.

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6. NREM Cortisol Dynamics Across The Night

Understanding NREM cortisol dynamics requires thinking about the sleep period not as a uniform block but as a progression of changing hormonal conditions across the night. NREM sleep comprises three stages (N1, N2, N3), each with distinct cortisol profiles that collectively orchestrate the nocturnal HPA axis suppression the body relies on for recovery.

Early Night: N3 Dominance And Cortisol Nadir

In the first 90–120 minutes of sleep, N3 slow-wave sleep is at its most abundant. This period corresponds almost exactly with the cortisol nadir — the lowest point in the 24-hour cortisol cycle. During this window, CRH pulse frequency decreases, ACTH secretion falls, and adrenal cortisol output drops to its daily minimum.

Sleep researchers sampling cortisol at frequent intervals across the night consistently find that plasma cortisol is lowest during the first two to three hours of sleep when N3 is most prevalent. This co-occurrence is not merely circadian — it is driven partly by the sleep process itself. Studies that acutely delay sleep onset (keeping participants awake later than usual) find that the cortisol nadir shifts with the delayed sleep onset, confirming that sleep — particularly deep sleep — actively suppresses cortisol beyond what circadian timing alone would predict.

Mid-Night Transition: N2 And Declining Suppression

As the first deep sleep block transitions to N2 in the middle of the night, cortisol suppression begins to ease slightly before the early morning rise. NREM cortisol suppression is most profound during N3 and somewhat less so during N2, though both stages are associated with significantly support healthy cortisol than either wakefulness or REM sleep.

N2 sleep spindles are thought to play a role in memory consolidation and sensory gating — essentially preventing external stimuli from disturbing sleep. Their frequency and density may influence how completely the sleeper transitions into and sustains N3 sleep, thereby indirectly affecting the depth and duration of NREM cortisol suppression.

Sleep Reactivity And NREM Structure

A 2023 PMC study on sleep reactivity provides important nuance to NREM cortisol dynamics. Under conditions of psychological stress, the study found that N1, N2, and REM durations decreased, while N3 sleep duration was relatively more stable. However, the same study found that high sleep-reactive participants — those whose sleep is most disrupted by stress — showed increased cortisol reactivity.

This suggests that the relationship between NREM sleep stages and cortisol is moderated by individual differences in stress reactivity. Some people's N3 sleep is relatively protected even under stress, but their cortisol response remains elevated regardless. Others may show both structural sleep architecture changes and cortisol dysregulation. These individual-difference factors are increasingly recognised as important targets for personalised sleep intervention.

7. The Deep Sleep HPA Axis Connection

The deep sleep HPA axis relationship is the master regulatory axis through which deep sleep exerts its hormonal restorative function. The HPA axis — comprising the hypothalamus, anterior pituitary, and adrenal cortex — is the body's primary stress response system, but it also serves as a critical mediator of sleep quality and hormonal rhythmicity.

HPA Axis Architecture And Sleep Staging

The hypothalamic CRH neurons that initiate HPA axis activation receive input from multiple sleep-related brain systems:

• The locus coeruleus (LC): A brainstem nucleus that is almost completely quiescent during N3 sleep, dramatically reducing noradrenaline release and consequently reducing HPA axis drive during deep sleep.
• The basolateral amygdala: A major stress-detection centre whose activity is markedly suppressed during N3 sleep, reducing emotionally-driven CRH release.
• Orexin/hypocretin neurons: These wakefulness-promoting neurons are inactive during sleep, removing a key HPA-activating drive during the sleep period.

The concerted inactivation of these arousal and stress-signalling systems during N3 sleep creates a neurobiological window in which HPA axis activity falls to its minimum — a window that the body uses for physical repair, immune defence, memory consolidation, and metabolic restoration.

What Happens When The HPA Axis Disrupts Deep Sleep

Clinical populations with HPA axis hyperactivation provide a natural experiment for understanding what happens when deep sleep HPA axis regulation fails. Patients with:

• Major depression: Characterised by HPA hyperactivation (elevated CRH, ACTH, and cortisol), and consistently show reduced N3 sleep, increased sleep fragmentation, and earlier REM onset.
• Cushing's syndrome: Pathological hypercortisolaemia produces severe insomnia, reduced SWS, and disrupted sleep architecture — effects that can be at least partially reversed with cortisol normalisation.
• Chronic insomnia disorder: Associated with HPA axis hyperactivation, elevated 24-hour cortisol profiles, and reduced slow-wave activity.
• Post-traumatic stress disorder (PTSD): Associated with HPA axis dysregulation, REM sleep fragmentation, nightmares, and severely disrupted sleep architecture.

Across all these conditions, normalising HPA axis function — whether through pharmacological, psychological, or behavioural interventions — is associated with improvements in deep sleep quality. And improving deep sleep quality, conversely, is associated with progressive HPA axis normalisation.

The Clinical Mirtazapine Evidence Revisited

The 2004 Nature study finding that mirtazapine treatment produced both increased SWS and a significant reduction in DEX-CRH test cortisol response is a clinically important demonstration of this bidirectional deep sleep HPA axis relationship. Mirtazapine's SWS-enhancing effects (likely mediated through H1 histamine blockade and serotonergic modulation) appear to facilitate HPA axis downregulation that is consistent with — and possibly mediated partly by — the restoration of normal deep sleep architecture.

8. Pre-Sleep Cortisol And Sleep Efficiency: What A 2024 Study Found

One of the most methodologically rigorous recent investigations into the sleep stages cortisol research literature comes from a 2024 intensive longitudinal study published in the Oxford Academic journal SLEEP. Its findings challenge some assumptions about how cortisol and sleep interact, and its methodology sets a new standard for this field.

Study Design And Methodology

Rather than relying on single-night lab assessments or between-person comparisons — the traditional approach in sleep-cortisol research — this 2024 study used a 15-day intensive longitudinal design with:

• 15 consecutive nights of EEG-assessed home sleep monitoring — providing objective, polysomnography-quality sleep staging rather than self-reported sleep quality
• 14 days of cortisol sampling — providing repeated measurements of the same individuals' cortisol fluctuations over time
• A within-person analytical framework — testing whether individuals sleep worse on their own high-cortisol nights compared to their own low-cortisol nights, rather than simply comparing high-cortisol people to low-cortisol people

This within-person design is critical because it controls for stable individual differences (personality, chronic stress level, typical sleep duration) that might otherwise confound the cortisol-sleep relationship.

Key Findings

The study's core findings were clear and clinically meaningful:

Higher pre-sleep cortisol predicted:

• Shorter total sleep time (TST): Participants slept fewer total minutes on nights when their pre-sleep cortisol was elevated relative to their personal average
• Lower sleep efficiency (SE): A greater proportion of time in bed was spent awake rather than asleep on high-cortisol nights
• Longer sleep onset latency (SOL): Participants took significantly longer to fall asleep when pre-sleep cortisol was elevated

No significant between-person effect: The study found no significant effect at the between-person level — meaning that simply having a generally higher average cortisol level did not robustly predict having generally worse sleep on average. The association was specifically at the within-person, day-to-day level.

Why The Day-To-Day Effect Matters

This finding has important practical implications. It means that the cortisol-sleep connection is not primarily a trait-level phenomenon — it is a state-level, daily dynamic. On any given night, the cortisol level you carry into bed predicts how well you will sleep that specific night, regardless of what your typical cortisol level is.

This places agency in the hands of individuals: because the effect operates day-to-day, interventions that support healthy cortisol before any specific night of sleep have the potential to meaningfully improve that night's sleep quality. And because the effect involves sleep efficiency, total sleep time, and sleep onset latency — the core metrics of restorative sleep — the implications for deep sleep architecture are direct.

When sleep onset is delayed by elevated cortisol, the early-night N3 deep sleep window is shortened. When sleep efficiency is reduced through fragmented awakenings, the consolidated deep sleep blocks required for full HPA axis downregulation are disrupted. The cortisol-sleep loop begins to tighten.

Implications For Sleep Architecture And Cortisol Research

The 2024 SLEEP study is important not only for its findings but for what it demonstrates methodologically. By combining EEG-assessed sleep staging with repeated cortisol biomarker sampling in a within-person framework, it demonstrates that the sleep architecture cortisol relationship is measurable, reproducible, and clinically meaningful at the level of individual nights. Future research using this paradigm will likely generate increasingly precise understanding of exactly which sleep stages are most affected by cortisol elevation, and exactly what threshold of pre-sleep cortisol begins to meaningfully impair deep sleep access.

9. How Stress Changes Sleep Architecture: 2026 Research

The 2026 ScienceDirect study on the impact of stress on sleep architecture represents one of the most recent additions to the cortisol and sleep stages research literature. Its findings confirm and extend earlier research while providing important new detail about the specific architectural shifts that occur under sustained daily stress.

Seven-Day Ecological Design

The study followed participants across a seven-day period in their natural environment — an ecological momentary assessment approach that captures real-world sleep architecture rather than the potentially atypical sleep produced in laboratory conditions. Daily stress levels were assessed alongside objective sleep architecture measurements across the week.

Key Findings: The Stress-Architecture Trade-Off

The study's central finding was that higher daily stress was associated with increased REM sleep percentage and decreased deep (N3) sleep percentage.

This finding is mechanistically coherent with the broader sleep architecture cortisol literature:

Why stress increases REM:

• Elevated cortisol and noradrenaline in the later half of the night (when REM dominates) create a neurochemical environment that extends and intensifies REM episodes
• Emotional memory processing demands — increased by stressful life events — may drive increased REM sleep as the brain attempts to process and integrate threatening experiences
• The amygdala hyperactivation associated with stress promotes the emotionally vivid, processing-oriented dreaming characteristic of stress-related REM increases

Why stress decreases N3:

• Elevated pre-sleep cortisol (documented in the 2024 SLEEP study) delays sleep onset, shrinking the early-night window when N3 is most abundant
• HPA axis hyperactivation creates arousal signals that interrupt the sustained, undisturbed sleep needed for deep slow-wave sleep consolidation
• Sleep fragmentation — increased under stress — prevents the uninterrupted 20+ minute N3 blocks that generate the deepest hormonal restoration

The Net Effect On Restorative Sleep Cortisol

The net result of this stress-driven architectural shift — more REM, less N3 — is profoundly counterproductive from a restorative sleep cortisol perspective. The body, already stressed and already carrying elevated cortisol, receives less of the deep sleep it most needs to clear cortisol, restore HPA axis tone, and build next-day stress resilience. Instead, it receives more of the lighter, emotionally active sleep that, while important in normal proportions, does not provide the same HPA axis suppression as slow-wave sleep.

This architectural trade-off may help explain why stressed individuals often report feeling unrested despite sleeping a normal number of hours: their sleep duration may be adequate but their sleep architecture is tilted away from the most restorative stages. Total sleep time is a poor proxy for sleep quality when the architectural composition of that sleep has been altered by stress hormones.

10. Deep Sleep Cortisol Clearance: What It Means For Recovery

Deep sleep cortisol clearance is a concept that captures one of the most clinically important functions of N3 slow-wave sleep: its role in reducing the cortisol load carried from one day into the next. Understanding this mechanism helps explain why consistently good deep sleep is not merely pleasant but physiologically essential for long-term stress resilience.

What Is Cortisol Clearance In The Sleep Context?

In the context of sleep biology, cortisol clearance refers to the processes by which cortisol levels accumulated during the day's stress responses are reduced during the sleep period, allowing waking cortisol to begin from a lower baseline and the HPA axis to respond to the next day's challenges with appropriate — rather than hyperactivated — reactivity.

Cortisol is metabolised primarily in the liver (by 5α- and 5β-reductases) and kidneys. Its metabolites are excreted in urine, which is why 24-hour urinary free cortisol is a standard clinical measure of HPA axis activity. During sleep — particularly deep sleep — several processes facilitate this clearance:

1. Reduced cortisol production: HPA axis suppression during N3 sleep reduces new cortisol secretion, allowing metabolic clearance to exceed production and plasma levels to fall.

1. Enhanced hepatic metabolism: Growth hormone, secreted in high quantities during N3, promotes hepatic function including the enzymatic metabolism of cortisol and its metabolites.

1. Reduced cortisol binding competition: During sleep, reduced metabolic demands and lower glucose mobilisation requirements mean that cortisol is less needed for its catabolic energy-mobilising functions, freeing it for metabolic degradation rather than tissue action.

1. Glymphatic activity: During deep sleep, the brain's glymphatic system — a recently discovered waste-clearance mechanism that operates primarily during NREM sleep — removes metabolic waste products including potentially neurotoxic substances. While cortisol itself is not a primary glymphatic target, this broad neural housekeeping function supports the neurobiological conditions that sustain healthy HPA axis regulation.

What Impaired Deep Sleep Cortisol Clearance Looks Like

When deep sleep is chronically shortened or fragmented — through insomnia, sleep apnoea, stress, shift work, or ageing — cortisol clearance is impaired. The downstream manifestations include:

• Elevated morning cortisol baseline: Each morning begins with more residual cortisol from the previous day, making the day's stress responses more intense.
• Flattened cortisol diurnal slope: The difference between morning peak and evening nadir cortisol shrinks, impairing the hormonal signalling that drives circadian rhythm icity.
• HPA axis sensitisation: With repeated nights of impaired cortisol clearance, the HPA axis becomes progressively sensitised — lower stress thresholds trigger disproportionately large cortisol responses.
• Allostatic load accumulation: The chronic elevation of stress hormone baseline contributes to the broader allostatic load — the cumulative biological cost of chronic stress — with implications for cardiovascular health, immune function, metabolic regulation, and cognitive ageing.

Protecting deep sleep is therefore not merely about feeling rested: it is about ensuring that the biological machinery of cortisol clearance operates each night, resetting the HPA axis for the next day's demands.

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11. Sleep Stages Cortisol Research: What EEG Studies Reveal

EEG-based sleep staging studies provide the most direct and objective evidence on cortisol and sleep stages research, moving beyond subjective reports and actigraphy to reveal exactly which sleep stages are associated with which hormonal states. The convergent findings from decades of polysomnographic cortisol research are remarkably consistent.

The Polysomnographic Evidence Base

Studies combining overnight polysomnography (PSG) with frequent cortisol sampling — typically via indwelling intravenous catheter or multiple salivary collections — have established several robust findings:

Cortisol Is Lowest During N3 Sleep Across multiple laboratories and diverse populations, plasma cortisol consistently reaches its lowest levels during N3 slow-wave sleep. This is true when controlling for clock time, meaning the suppression is attributable to the sleep stage itself, not merely the time of night.

Sleep Fragmentation Elevates Cortisol Experimental induction of sleep fragmentation (through auditory stimuli or other disturbances) — while preserving total sleep time — elevates nocturnal cortisol levels compared to unfragmented sleep. This demonstrates that sleep continuity, not merely sleep duration, is necessary for optimal cortisol clearance.

First Sleep Cycle Is Most Critical For HPA Suppression Studies examining cortisol secretion episode timing find that the most pronounced HPA axis suppression occurs during the first two sleep cycles, when N3 predominates. Later sleep cycles, even when they include some N3, provide less dramatic cortisol suppression because baseline cortisol has already begun its pre-waking rise.

EEG Delta Power Negatively Correlates With Cortisol Studies quantifying EEG delta power (the strength of slow-wave activity, not merely its presence) find that greater delta power is associated with lower concurrent cortisol levels. This dose-response relationship suggests that the depth of slow-wave sleep matters for cortisol suppression, not just its presence.

The 2024 SLEEP Study's EEG Methodology

The 2024 Oxford Academic study's use of 15 consecutive nights of EEG-assessed home sleep is notable in the cortisol and sleep stages research context because it demonstrates that the cortisol-sleep relationship is detectable not only in controlled laboratory conditions but in participants' natural home environments. The ecological validity of EEG-assessed sleep staging has historically limited clinical sleep-cortisol research, which has tended to rely on lab-based PSG. The 2024 study's methodology suggests that ambulatory EEG assessment combined with salivary cortisol sampling offers a practical approach for larger-scale naturalistic studies — and potentially for clinical monitoring of stress-related sleep disruption.

What Sleep Stage Is Most Affected By Stress?

Drawing together the EEG evidence, the answer to this frequently asked question is nuanced:

• N3 deep sleep is most impaired in terms of proportion and quality under chronic stress conditions, particularly when pre-sleep cortisol is elevated
• REM sleep is increased in proportion under acute stress but may become fragmented and less restorative under chronic stress
• N1 and N2 show more modest stress-related changes in most studies, though the 2023 PMC study on sleep reactivity found that high sleep-reactive individuals showed decreases in N1 and N2 durations under stress — suggesting that sleep fragmentation under stress may reduce all NREM stages while disproportionately redistributing time toward and then away from REM

The consensus picture: stress and elevated cortisol most severely compromise the N3 deep sleep that is most essential for hormonal restoration, while initially increasing — and ultimately disrupting — REM sleep emotional processing.

12. Does Insomnia Cause High Cortisol, Or Does Cortisol Cause Insomnia?

This is one of the most common questions in the sleep architecture cortisol deep sleep literature, and the honest answer is: both, and the direction of causality is difficult to fully disentangle. The relationship between insomnia and cortisol is bidirectional, with multiple positive feedback loops that can perpetuate both conditions simultaneously.

Evidence That High Cortisol Causes Insomnia

The 2024 Oxford Academic study provides direct within-person evidence that elevated pre-sleep cortisol precedes and predicts worse sleep on the same night — longer sleep onset latency, lower sleep efficiency, and shorter total sleep time. Since cortisol was measured before sleep and sleep was measured afterwards, the temporal sequence supports cortisol as a causal driver of that night's sleep quality.

Mechanistically, elevated cortisol and its upstream driver CRH have direct arousal-promoting, sleep-inhibiting effects:

• CRH infusion in experimental animals reliably produces wakefulness and suppresses sleep
• Cortisol increases central nervous system arousal, making it harder to disengage from waking cognitive activity
• HPA axis hyperactivation disrupts the circadian suppression of cortisol in the evening, maintaining arousal signals into the sleep period

Evidence That Insomnia Causes High Cortisol

Conversely, experimental and clinical studies consistently show that poor sleep elevates subsequent cortisol:

• Sleep restriction studies (limiting participants to 4–6 hours per night) produce significant elevations in afternoon and evening cortisol within days
• Selective suppression of N3 sleep without reducing total sleep time elevates morning cortisol
• Chronic insomnia disorder is associated with elevated 24-hour urinary free cortisol, elevated evening cortisol, and HPA axis hyperreactivity to standardised stress tests
• Patients with insomnia show reduced cortisol suppression in response to low-dose dexamethasone — suggesting genuine HPA axis dysregulation, not merely normal responses to a stressful situation

The Perpetuating Cycle

The most clinically important insight is not which causes which but how the cycle perpetuates:

Night 1: Stress → elevated pre-sleep cortisol → shorter N3 sleep, longer sleep onset, lower sleep efficiency

Day after Night 1: Sleep debt → impaired cortisol clearance → elevated daytime and evening cortisol → greater stress reactivity

Night 2: Higher baseline cortisol → further disrupted sleep architecture → even less N3 deep sleep

Day after Night 2: Progressive HPA sensitisation → increasing allostatic load → worsening pre-sleep cortisol elevation

Without intervention, this cycle can become chronic insomnia with comorbid HPA axis dysregulation — a condition substantially more difficult to treat than either acute stress-related sleep disruption or episodic insomnia.

The practical implication: interventions need to address both sides of the equation simultaneously. Cortisol-lowering strategies alone will improve sleep if the cycle is primarily cortisol-driven, but sleep improvements alone may not fully normalise cortisol if the HPA axis has been chronically sensitised. The most effective approaches target both deep sleep restoration and HPA axis downregulation in parallel.

13. Which Sleep Stage Is Most Vulnerable To Stress?

Based on the convergent evidence from sleep architecture cortisol research — including the 2024 Oxford Academic study, the 2026 ScienceDirect study, and the 2023 PMC sleep reactivity research — a clear picture emerges of relative sleep stage vulnerability to psychological and physiological stress.

N3 Deep Sleep: Most Impaired In Proportion

The evidence consistently shows that N3 deep sleep bears the greatest proportional reduction under sustained daily stress. The 2026 seven-day ecological study found that higher daily stress was associated with decreased deep (N3) sleep percentage — a finding that aligns with the broader experimental literature on cortisol-induced sleep architecture changes.

N3 is vulnerable for several reasons:

• It requires sustained, uninterrupted sleep — a condition that high cortisol directly undermines through increased arousal and fragmentation
• It is front-loaded in the sleep period, making it the first casualty when sleep onset is delayed by elevated pre-sleep cortisol
• Its depth (measured by delta power) is sensitive to acute cortisol elevation, meaning that even when N3 occurs, its quality may be diminished under high-cortisol conditions

REM Sleep: Increased Initially, Disrupted Chronically

Counterintuitively, acute and moderate stress appears to increase REM sleep proportion — as the 2026 study confirmed, higher daily stress was associated with increased REM sleep percentage. This may reflect increased demand for emotional memory processing and threat appraisal under stress.

However, this is not straightforwardly beneficial. Increased REM proportion under stress tends to involve more fragmented, less consolidated REM episodes — sometimes manifest as more frequent, emotionally intense dreaming and more awakenings from REM. In chronic stress and PTSD, REM fragmentation becomes severe, with REM onset occurring unusually early in the sleep period (reduced REM latency) and nightmare-related awakenings disrupting the second half of the night.

N1 And N2: Variable Vulnerability

The 2023 PMC sleep reactivity study found that N1, N2, and REM durations decreased under stress in high sleep-reactive individuals — suggesting that in people with greater sleep vulnerability, stress may actually reduce all stage durations, possibly because increased wakefulness after sleep onset consumes time that would otherwise be spent in sleep stages. In lower sleep-reactive individuals, N3 may be more specifically affected while N1 and N2 remain relatively preserved.

Individual Differences In Stage Vulnerability

The 2023 PMC study's finding that high sleep-reactive individuals showed increased cortisol reactivity under stress, alongside greater sleep architecture disruption, highlights the importance of individual differences. Not all people experience the same pattern of stage-specific vulnerability. Factors including trait anxiety, HPA axis reactivity, circadian chronotype, and pre-existing sleep disorders all modulate which stages are most affected in any given individual.

14. Interventions That Improve Sleep Architecture And Support healthy cortisol

Given the bidirectional deep sleep HPA axis relationship, interventions that improve restorative sleep cortisol dynamics need to work on both sides of the equation: protecting and enhancing N3 deep sleep while also reducing the cortisol and HPA axis hyperactivation that disrupts it.

Cognitive Behavioural Therapy For Insomnia (CBT-I)

CBT-I is the gold-standard first-line treatment for chronic insomnia and has the strongest evidence base for improving sleep architecture. Key components include stimulus control, sleep restriction therapy, cognitive restructuring of sleep-related beliefs, and relaxation training. CBT-I has been shown to:

• Increase sleep efficiency and reduce sleep onset latency — the same metrics impaired by elevated pre-sleep cortisol in the 2024 SLEEP study
• Improve subjective and objective deep sleep quality
• Reduce 24-hour cortisol output in patients with chronic insomnia
• Decrease HPA axis hyperreactivity over the course of treatment

Mindfulness-Based Stress Reduction (MBSR)

Evening Cortisol Management

Since the 2024 study demonstrated that pre-sleep cortisol specifically predicts that night's sleep quality, interventions targeting cortisol in the hours before bed have direct relevance:

• Light exposure management: Blue-light exposure in the evening suppresses melatonin and maintains cortisol — using blue-light blocking glasses or avoiding screens 1–2 hours before bed can facilitate the normal evening cortisol decline.
• Evening exercise timing: Vigorous exercise in the 2–3 hours before bed can elevate cortisol and body temperature, delaying sleep onset. Earlier exercise timing (morning or midday) is preferable for sleep architecture preservation.
• Caffeine restriction: Caffeine's adenosine antagonism delays sleep onset and reduces slow-wave sleep — combining with its mild cortisol-elevating effect, it is doubly disruptive to deep sleep architecture.
• Evening relaxation practices: Progressive muscle relaxation, yoga nidra, restorative yoga, and breathing practices (particularly extended exhalation techniques) activate the parasympathetic nervous system, reducing sympathetic HPA axis drive in the pre-sleep period.

Pharmacological Approaches

The mirtazapine evidence from the 2004 Nature study provides a pharmacological proof-of-concept for the deep sleep-HPA axis therapeutic strategy. By enhancing SWS, mirtazapine appears to facilitate the HPA axis normalisation measured by the DEX-CRH test. Other pharmacological approaches with evidence for SWS enhancement and potential cortisol normalisation include:

• Low-dose trazodone: Enhances slow-wave sleep without the residual sedation of benzodiazepines
• Melatonin: Supports circadian alignment and mildly promotes NREM sleep, with some evidence for cortisol reduction
• Gamma-hydroxybutyrate (sodium oxybate): A powerful SWS promoter used in narcolepsy, with robust cortisol-reducing effects — limited by its controlled substance status and side effect profile

Addressing Sleep Disordered Breathing

Obstructive sleep apnoea (OSA) — the most prevalent sleep disorder — severely fragments sleep and suppresses N3 deep sleep through repeated micro-arousals. OSA is also strongly associated with HPA axis dysregulation and elevated cortisol. CPAP treatment of OSA consistently improves sleep architecture (increasing SWS and REM sleep quality) and reduces markers of HPA axis activation. In individuals with undiagnosed OSA presenting with insomnia symptoms and stress reactivity, treating the underlying sleep disordered breathing may be more impactful than any other single intervention.

Environmental Optimisation

The sleep environment plays an underappreciated role in protecting deep sleep architecture:

• Temperature: Core body temperature must drop 1–2°C for optimal deep sleep; bedroom temperature of 16–19°C (60–67°F) supports this
• Darkness: Complete darkness during sleep prevents the light-induced cortisol elevation that can disrupt the sleep period
• Noise management: Even sub-awakening noise levels (40–55 dB) can reduce slow-wave delta power without producing conscious awakening

15. Biomarkers For Tracking Sleep Quality Beyond Subjective Reports

As cortisol and sleep stages research has demonstrated, subjective sleep quality reports are an imperfect and sometimes misleading index of actual sleep architecture. Developing valid, accessible biomarkers for sleep quality — including deep sleep specifically — is an active area of research with significant clinical implications.

Salivary Cortisol

Salivary cortisol measurement has become the research standard for repeated ecological cortisol assessment, as used in the 2024 SLEEP study with its 14-day sampling protocol. For clinical and self-monitoring purposes:

• Morning cortisol awakening response (CAR): Can be self-sampled with simple salivary collection kits at +0, +15, +30, and +45 minutes post-waking. The CAR amplitude and total area under the curve correlates with HPA axis reactivity and indirectly reflects preceding sleep quality.
• Evening salivary cortisol: Elevated evening cortisol (>5 nmol/L in evening samples) predicts worse that night's sleep and can serve as a warning indicator for pre-sleep arousal management.

EEG-Based Sleep Staging

Home-based EEG sleep monitoring devices have improved dramatically in accuracy and accessibility. Consumer-grade headbands using frontal EEG electrodes can now provide reasonably accurate N1/N2/N3/REM staging, delta power quantification, and sleep spindle detection. The clinical-grade EEG assessment used in the 2024 SLEEP study demonstrates the standard for research, while consumer devices like the Dreem headband and similar products approach this standard for everyday use.

Heart Rate Variability (HRV)

HRV — the beat-to-beat variation in heart rate — is a surrogate marker of autonomic nervous system balance. Higher HRV reflects greater parasympathetic tone and is associated with better deep sleep quality, support healthy cortisol, and greater HPA axis resilience. HRV is easily measured by consumer wearables (Polar chest straps, Apple Watch, Whoop, Garmin, Oura Ring) and can serve as a proxy indicator of deep sleep quality:

• Nocturnal HRV peaks during deep sleep and falls during REM
• Morning HRV (first full minute after waking) reflects the preceding night's deep sleep quality and autonomic restoration
• HRV trend over multiple days tracks cumulative sleep debt and HPA axis strain better than single-night measures

Urinary Free Cortisol (UFC)

24-hour urinary free cortisol measurement captures total cortisol excretion and is less sensitive to the timing fluctuations that affect single blood or salivary samples. Chronically elevated UFC in the context of poor sleep quality and stress symptoms is a strong indicator of sustained HPA axis hyperactivation that warrants targeted intervention.

Actigraphy And Wrist-Based Sleep Tracking

While wrist actigraphy cannot distinguish sleep stages, it provides validated measurement of total sleep time, sleep efficiency, and sleep onset latency — the exact metrics found to be impaired by elevated pre-sleep cortisol in the 2024 SLEEP study. For day-to-day monitoring of sleep quality trends, validated actigraphy offers a practical and accessible tool that captures the macro-level sleep architecture changes driven by cortisol and stress.

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16. Key Takeaways And Practical Summary

The relationship between sleep architecture and cortisol deep sleep biology is now supported by a robust and growing body of research. Here is a distilled summary of the most important findings and their practical implications.

What The Research Establishes

1. Deep sleep is the primary hormonal restoration stage. N3 slow-wave sleep is associated with cortisol's nadir, HPA axis suppression, peak growth hormone secretion, and the deepest restorative sleep architecture. Protecting N3 sleep is not a luxury — it is a biological necessity for hormonal homeostasis.

2. Pre-sleep cortisol predicts that night's sleep quality — day by day. The 2024 Oxford Academic SLEEP study demonstrated that within individuals, higher pre-sleep cortisol predicted shorter total sleep time, lower sleep efficiency, and longer sleep onset latency on a night-by-night basis. Managing evening cortisol levels is directly relevant to that night's sleep architecture.

3. Stress shifts sleep architecture from deep to REM sleep. The 2026 ScienceDirect seven-day study confirmed that higher daily stress is associated with increased REM percentage and decreased N3 percentage — a trade-off that reduces restorative sleep cortisol clearance precisely when it is most needed.

4. The deep sleep-cortisol relationship is bidirectional. High cortisol disrupts deep sleep. Disrupted deep sleep impairs cortisol clearance. This bidirectional loop can perpetuate stress-sleep dysfunction without intervention.

5. Slow-wave sleep actively suppresses HPA axis activity. The clinical evidence — including the mirtazapine study's finding of parallel SWS increases and DEX-CRH cortisol reductions — supports the concept that slow-wave sleep is not merely correlatedwith low cortisol but actively drives HPA axis downregulation.

6. Individual differences matter. High sleep-reactive individuals show greater cortisol reactivity under stress alongside greater sleep architecture disruption. Interventions may need to be tailored to individual stress-sleep phenotypes.

7. EEG-assessed sleep staging provides objective evidence unavailable from subjective reports. Research combining polysomnographic staging with cortisol biomarkers has generated the most definitive evidence on this topic — and consumer-grade EEG tools are making this level of assessment increasingly accessible outside the laboratory.

Practical Action Points

Based on the converging research, the most evidence-supported strategies for protecting sleep architecture and managing restorative sleep cortisol dynamics include:

• Manage evening cortisol: Avoid intense exercise, bright light, and stimulants in the 2–3 hours before bed. Implement relaxation practices specifically targeting pre-sleep cortisol reduction.
• Protect early-night sleep: The first 90–120 minutes of sleep contain the most critical N3 deep sleep. Sleep onset delays caused by elevated pre-sleep cortisol disproportionately cost you the most restorative portion of the night.
• Prioritise sleep consistency: Regular sleep and wake times support circadian HPA axis rhythmicity, ensuring the normal evening cortisol decline occurs reliably.
• Consider CBT-I for chronic sleep problems: If stress-related sleep disruption has become chronic, CBT-I offers the strongest evidence base for restoring sleep architecture and normalising cortisol.
• Monitor objective sleep quality: EEG-based home sleep assessment, HRV measurement, and morning salivary cortisol sampling provide objective feedback on whether interventions are improving your sleep architecture and HPA axis function.
• Address sleep disordered breathing: Undiagnosed OSA severely suppresses deep sleep and elevates cortisol — and is more common than widely recognised. If you snore, wake unrefreshed, or feel excessively sleepy, seek evaluation.
• Treat stress and sleep together: The bidirectional cortisol-sleep loop means that addressing only one side of the equation is less effective than integrated approaches that simultaneously support HPA axis downregulation and deep sleep restoration.

Frequently Asked Questions

Q: What is the relationship between cortisol and deep sleep? Cortisol and deep sleep have a reciprocal inhibitory relationship. Deep sleep (N3) suppresses cortisol through HPA axis downregulation, while elevated cortisol disrupts deep sleep through arousal-promoting mechanisms. Cortisol reaches its 24-hour nadir during peak N3 slow-wave sleep in the early night.

Q: Does high nighttime cortisol reduce slow-wave sleep? Yes. Both experimental manipulation studies and naturalistic within-person research (including the 2024 SLEEP study) confirm that higher pre-sleep and nighttime cortisol is associated with reduced sleep efficiency and disrupted sleep architecture, with N3 deep sleep being most affected.

Q: Can stress change sleep architecture? Yes. The 2026 ScienceDirect study demonstrated that higher daily stress was associated with increased REM sleep percentage and decreased deep (N3) sleep percentage across a seven-day period, confirming that stress produces measurable, objective changes in sleep stage distribution.

Q: Can improving sleep support healthy cortisol levels the next day? Yes. Studies show that improving deep sleep quality — whether through CBT-I, pharmacological SWS enhancement (as with mirtazapine), or sleep hygiene optimisation — reduces the next morning's cortisol levels and over time normalises HPA axis reactivity.

Q: What do EEG studies show about cortisol and sleep staging? EEG-based studies consistently show that cortisol is lowest during N3 slow-wave sleep, that EEG delta power negatively correlates with concurrent cortisol levels, and that sleep fragmentation — even without reducing total sleep time — elevates cortisol. The 2024 SLEEP study's use of 15 consecutive nights of EEG assessment demonstrated these relationships in ecologically valid home settings.

References And Further Reading

1. Held, K., et al. (2004). Mirtazapine increases non-REM sleep, growth hormone secretion, and sympathetic activity while reducing cortisol secretion in healthy males. Nature, 430. https://www.nature.com/articles/1300923

1. Slavish, D.C., et al. (2024). Daily associations between pre-sleep cortisol and objective sleep: A 15-night intensive longitudinal study. SLEEP, 47(9), zsae087. Oxford Academic. https://academic.oup.com/sleep/article/47/9/zsae087/7642187

1. [Author(s)] (2026). The impact of stress on sleep architecture: A seven-day study. ScienceDirect. https://www.sciencedirect.com/science/article/pii/S2352721826000197

1. PMC (2023). Sleep reactivity, stress, and cortisol: Examining sleep macrostructure changes. PubMed Central.

This article is for educational and informational purposes. It does not constitute medical advice. If you are experiencing significant sleep problems, chronic stress, or suspect HPA axis dysregulation, consult a qualified healthcare professional.