Lack of sleep cortisol: What the evidence shows

Tom
2026-04-26T11:02:16+01:00

Even a single night of poor sleep can measurably alter your cortisol rhythm. One scoping review found that shift workers — people with chronically disrupted sleep — show blunted morning cortisol peaks and flattened diurnal slopes compared to day workers. That flattening matters. Your cortisol curve is not just a stress signal; it is a timing signal for nearly every system in your body.

In this article

1. What the evidence actually shows

2. The biological mechanism: why sleep loss shifts your cortisol curve

3. Light exposure, circadian rhythm, and the cortisol connection

4. Sleep disorders, cortisol, and what the clinical data shows

5. Melatonin, cortisol, and the hormonal see-saw

6. What you can actually do: the evidence-supported options

7. Frequently asked questions

8. My honest take

What the evidence actually shows

The relationship between sleep disruption and cortisol dysregulation is one of the better-documented areas in sleep science, though the direction of causality is still being worked out. Grosser et al. (2022) conducted a scoping review of cortisol and shift work, finding consistent evidence that disrupted sleep-wake cycles alter cortisol secretion patterns — specifically, a flattening of the diurnal slope that is associated with poorer health outcomes. The review covered studies across multiple industries and countries, making it one of the more generalisable summaries available.

Hayton et al. (2024) reported something that struck me as particularly useful: in their short report, a lack of normal diurnal variation in salivary cortisol was directly linked to sleep disturbances and heightened anxiety scores. That is not just "stress causes bad sleep." It is a bidirectional signal — disrupted cortisol rhythm may contribute to the sleep problems, and the sleep problems may further disrupt the cortisol rhythm. The loop feeds itself.

Boivin et al. (2022) mapped the circadian system disruption in shift workers in granular detail, showing how misalignment between the internal clock and external light-dark cycles cascades into hormonal dysregulation — cortisol included. The data here is not from small convenience samples; it draws on decades of circadian biology research. I find this paper useful precisely because it does not overstate. It acknowledges that individual variability is substantial.

The biological mechanism: why sleep loss shifts your cortisol curve

Cortisol follows a predictable 24-hour rhythm under normal conditions. It peaks roughly 30 to 45 minutes after waking — the cortisol awakening response — then declines steadily across the day, reaching its lowest point in the early hours of the night. This rhythm is driven by the hypothalamic-pituitary-adrenal (HPA) axis, which is itself entrained by the suprachiasmatic nucleus (SCN) in the hypothalamus — your central circadian clock.

Sleep deprivation disrupts this system at multiple points. First, it elevates evening cortisol. Studies suggest that even partial sleep restriction — losing two to three hours per night — may shift cortisol secretion later into the evening, at exactly the time it should be falling. Elevated evening cortisol then makes it harder to fall asleep, because cortisol is an arousal signal. You can see why this becomes self-reinforcing.

Second, sleep loss activates the HPA axis more generally. The brain interprets insufficient sleep as a physiological stressor, which may trigger increased corticotropin-releasing hormone (CRH) from the hypothalamus, driving adrenocorticotropic hormone (ACTH) release from the pituitary, which in turn signals the adrenal cortex to produce more cortisol. This is the same pathway activated by psychological stress, illness, or caloric restriction.

Third — and this is the part that often gets overlooked — the timing of cortisol matters as much as the total amount. A blunted morning peak means you may feel less alert at the start of the day. A flattened diurnal slope, as documented by Grosser et al. (2022), is associated in observational data with a range of downstream effects on metabolic and immune function, though the causal direction in humans is not always clean.

Light exposure, circadian rhythm, and the cortisol connection

One underappreciated driver of the lack-of-sleep cortisol relationship is light — specifically, the absence of it at the right times. Harb et al. (2015) found that workers without access to natural light in their workspace reported significantly worse sleep quality, more depressive symptoms, and disrupted physiological rhythms compared to those with window exposure. The effect on cortisol rhythm was part of that picture.

Morning light is the primary zeitgeber — the external time cue — that anchors your SCN to the 24-hour day. Without it, your internal clock drifts. When the clock drifts, cortisol timing drifts with it. Evening light exposure compounds this by suppressing melatonin, which delays sleep onset, which shortens total sleep time, which — as the evidence suggests — may push cortisol higher at the wrong times.

This is not abstract. If you work in an office with no windows, commute before dawn, and spend your evenings under bright artificial light, you are stacking multiple circadian disruption signals. The cortisol data from shift work research — like that summarised in Boivin et al. (2022) — is probably the closest proxy we have for what happens to people living this way, even if they are not technically "shift workers."

Sleep disorders, cortisol, and what the clinical data shows

It is worth separating chronic mild sleep restriction from clinical sleep disorders, because the cortisol data looks somewhat different across these populations.

Obstructive sleep apnoea

In obstructive sleep apnoea (OSA), repeated nocturnal hypoxia and arousal events create a pattern of fragmented, non-restorative sleep. Ken-Dror et al. (2021) conducted a meta-analysis examining cortisol levels in OSA patients treated with continuous positive airway pressure (CPAP). The data suggested that CPAP treatment may reduce cortisol levels in this population, implying that the sleep fragmentation itself was contributing to elevated cortisol — not just the hypoxia. Sample sizes across the included studies were modest, and the authors noted significant heterogeneity, so I would not overread the effect sizes. But the direction of the finding is consistent with the broader mechanistic picture.

Narcolepsy and wake-system dysregulation

Shan et al. (2023) examined activated wake systems in narcolepsy type 1, a condition characterised by orexin deficiency and severely disrupted sleep architecture. The research highlights how wake-promoting neurochemical systems interact with HPA axis activity — a reminder that the cortisol-sleep relationship is not simply about "how many hours you slept" but about the quality and architecture of that sleep.

Melatonin, cortisol, and the hormonal see-saw

Melatonin and cortisol operate on roughly opposing schedules. Melatonin rises in darkness and signals sleep onset; cortisol rises at dawn and signals waking. When sleep is disrupted, both rhythms can shift out of their normal phase relationship.

Strassman et al. (1989) investigated whether exogenous melatonin acutely modulates cortisol secretion at night. Their finding: it does not appear to, at least not in a direct, immediate sense. This is an older study and the methodology has limitations, but it is relevant because it suggests that the melatonin-cortisol relationship is more about circadian phase than about one hormone directly suppressing the other. Supplementing melatonin to shift your sleep timing may indirectly affect cortisol rhythm over time — but it is not a direct cortisol-lowering intervention.

Meth et al. (2023) found that a weighted blanket may increase pre-sleep salivary melatonin concentrations in young healthy adults. This is a small, preliminary finding — I would not hang too much on it — but it points to the idea that relatively simple environmental interventions might influence the hormonal conditions around sleep onset. Whether this translates to meaningful cortisol changes downstream has not been adequately studied.

What you can actually do: the evidence-supported options

I am going to be direct here. There is no supplement that "fixes" a disrupted cortisol rhythm caused by inadequate sleep. The primary intervention is sleep itself. That said, there are some areas where the evidence is worth knowing about.

Glycine

Glycine is an amino acid with some preliminary human data suggesting it may support sleep quality when taken before bed. Research is ongoing, and large-scale human trials are limited — I would be overstating it to claim this is settled science. The KōJō Daily Formula includes 2,000mg of crystalline glycine, which aligns with the doses used in the available small-scale studies. Whether it meaningfully influences cortisol rhythm is not something the current evidence supports claiming.

Taurine

Taurine has been studied for its potential role in neurological function and as a modulator of the stress response in animal models. Large-scale human trials specifically examining taurine and cortisol are limited, and I would not overstate what the current evidence shows. Research is ongoing.

Vitamin C

Vitamin C contributes to the reduction of tiredness and fatigue — that is a registered claim, and it is one I can make without hedging. Vitamin C also contributes to the protection of cells from oxidative stress. Sleep deprivation is associated with increased oxidative stress markers in some studies, which is one reason adequate micronutrient intake matters during periods of poor sleep. The KōJō Daily Formula includes 500mg of crystalline Vitamin C.

Cognitive function and sleep-deprived states

If you are interested in how cognitive performance holds up under sleep pressure — and how certain compounds may support it — the evidence around nootropic ingredients is worth reading separately. I have written about this in more depth in the nootropic supplement UK piece, which covers the primary literature on cognition-supporting compounds with a similarly sceptical eye.

Environmental and behavioural factors

The data on light exposure is probably stronger than any supplement evidence. Getting bright light within 30 minutes of waking, reducing artificial light in the two hours before bed, and keeping a consistent sleep-wake time — including weekends — are the interventions with the most consistent mechanistic and observational support. Harb et al. (2015) makes a compelling case for natural light access as a meaningful variable, not just a lifestyle preference.

Frequently asked questions

Does lack of sleep directly cause high cortisol?

The relationship is more nuanced than "less sleep equals more cortisol." Sleep deprivation may alter cortisol timing — flattening the morning peak and elevating evening levels — rather than simply raising total output. Grosser et al. (2022) document this pattern in shift workers across multiple studies. The direction of effect and individual variability are both significant factors.

Can improving sleep quality lower cortisol levels?

Some clinical evidence suggests it may. Ken-Dror et al. (2021) found in a meta-analysis that CPAP treatment for sleep apnoea was associated with reduced cortisol levels in affected patients. Whether this generalises to healthy people with mild sleep restriction is not yet clear from the available data.

Is the cortisol awakening response affected by poor sleep?

Research suggests it may be. Hayton et al. (2024) found that a lack of normal diurnal cortisol variation — which includes the awakening response — was linked to sleep disturbances and heightened anxiety. A blunted awakening response is one of the more consistent findings in sleep-disrupted populations, though effect sizes vary considerably between studies.

Does melatonin supplementation reduce cortisol?

The evidence does not support melatonin as a direct cortisol-lowering agent. Strassman et al. (1989) found no acute modulatory effect of melatonin on nocturnal cortisol secretion. Melatonin may help shift sleep timing, which could indirectly influence cortisol rhythm over time — but that is a different claim, and the long-term data in healthy adults is limited.

How does shift work affect the cortisol-sleep relationship?

Shift work represents one of the most studied models of chronic sleep-circadian disruption. Boivin et al. (2022) detail how misalignment between internal clock timing and external light-dark cycles cascades into hormonal dysregulation. The cortisol changes observed in shift workers are among the more consistent findings in this field, though individual differences remain large.

Can anxiety worsen the sleep-cortisol cycle?

It appears so. Hayton et al. (2024) found that disrupted cortisol diurnal variation was linked to both sleep disturbances and heightened anxiety simultaneously. This suggests a three-way interaction rather than a simple linear cause-and-effect — anxiety, cortisol rhythm, and sleep quality may each influence the others in ways that are difficult to disentangle in observational data.

My honest take

I started reading the cortisol-sleep literature expecting a clean story. I did not find one. The evidence is consistent in its direction — sleep disruption alters cortisol rhythm, and disrupted cortisol rhythm makes sleep harder — but the effect sizes vary enormously between studies, individual responses differ substantially, and the causal arrows are messier than most health content lets on.

What I do believe, based on the weight of the evidence, is that the timing of cortisol matters more than most people realise. A flat diurnal slope is not just a curiosity. It shows up repeatedly in populations with poor health outcomes, though whether it is a cause or a marker is still being worked out. The shift work data from Grosser et al. (2022) and Boivin et al. (2022) is, to my mind, the most useful window into what happens when sleep and circadian rhythm are chronically misaligned — even for people who would never describe themselves as shift workers.

The supplement angle is where I want to be careful. I make a formula that includes glycine and taurine, both of which have been studied in the context of sleep and stress. But I am not going to tell you they fix a cortisol problem caused by insufficient sleep. The evidence does not support that, and I would rather lose a sale than overstate what the science shows.

The most evidence-consistent interventions remain the unglamorous ones: consistent sleep and wake times, morning light exposure, reduced artificial light in the evening, and — if you suspect a clinical sleep disorder — getting it properly assessed. Everything else is secondary. I think that is worth saying plainly, even when you are running a supplement brand.

This article is for informational purposes only and does not constitute medical advice. Consult your healthcare provider before starting any supplement regimen.

References (10 studies)

1. Grosser et al. (2022) — Cortisol and shiftwork: A scoping review. PMID 35872400.

2. Hayton et al. (2024) — Short Report: Lack of Diurnal Variation in Salivary Cortisol Is Linked to Sleep Disturbances and Heightened Anxiety. PMID 36975245.

3. Boivin et al. (2022) — Disturbance of the Circadian System in Shift Work and Its Health Impact. PMID 34969316.

4. Ken-Dror et al. (2021) — Changes in cortisol levels by continuous positive airway pressure in patients with obstructive sleep apnoea: Meta-analysis. PMID 34323304.

5. Harb et al. (2015) — Lack of exposure to natural light in the workspace is associated with physiological, sleep and depressive symptoms. PMID 25424517.

6. Shan et al. (2023) — Activated Wake Systems in Narcolepsy Type 1. PMID 37395722.

7. Strassman et al. (1989) — Lack of an acute modulatory effect of melatonin on human nocturnal thyrotropin and cortisol secretion. PMID 3211283.

8. Meth et al. (2023) — A weighted blanket increases pre-sleep salivary concentrations of melatonin in young, healthy adults. PMID 36184925.

9. Salgueiro et al. (2021) — Effect of Photobiomodulation on Salivary Cortisol, Masticatory Muscle Strength, and Clinical Signs in Children with Sleep Disorders. PMID 32990504.

10. Yamaguchi et al. (2023) — Exploring role of sleep aids in sleep problems in preschool children. PMID 37095165.

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