Cortisol and sleep: What evidence actually shows

Tom
2026-04-28T11:03:22+01:00

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Cortisol levels and sleep: What the evidence shows

Cortisol and sleep are locked in a bidirectional relationship. Poor sleep dysregulates cortisol; elevated cortisol disrupts sleep architecture. The research is clear on the mechanism. What works to rebalance both is less clear — and I'll walk through the honest gaps.

In this article

1. What the evidence actually shows

2. How cortisol and sleep are wired together

3. Sleep timing and cortisol: the circadian mismatch problem

4. What the supplement evidence actually supports

5. The role of consistency and sleep hygiene

6. Stress reduction and the cortisol-sleep loop

7. Age, cortisol, and sleep architecture changes

8. Clinical populations: opioid use disorder and trauma recovery

9. Frequently asked questions

10. My honest take

What the evidence actually shows

Let me start with the core finding: Wright et al. (2016) demonstrated that sleep deprivation and circadian misalignment raise cortisol levels and shift inflammatory markers in measurable ways. That's the bidirectional part. But the reverse is equally true — elevated cortisol at night suppresses melatonin and fragments sleep architecture.

Morgan et al. (2017) found that in older adults, those with shorter sleep duration showed significantly higher daytime cortisol levels. The effect wasn't subtle: individuals sleeping fewer than six hours per night had cortisol concentrations that remained elevated throughout the day, whereas those sleeping seven to eight hours showed the normal cortisol decline curve.

The practical question most people ask me is: can you supplement your way out of this? The answer is more nuanced than the supplement industry suggests. Sladek et al. (2021) found that daily rumination about stress — not just stress itself — predicted elevated diurnal cortisol activity. This tells me that the cortisol-sleep problem is partly neurobiological and partly behavioural. A supplement alone won't address the rumination piece.

How cortisol and sleep are wired together

Your cortisol follows a circadian rhythm. It peaks around 30 minutes after waking, then declines steadily through the day, hitting its nadir around midnight. This pattern is controlled by your suprachiasmatic nucleus — the brain's master clock — and reinforced by light exposure and sleep-wake timing.

When you sleep poorly or irregularly, two things happen. First, your circadian rhythm desynchronises. The cortisol peak flattens or occurs at the wrong time of day. Second, your body interprets fragmented sleep as a mild stressor, so cortisol doesn't decline as far at night. You end up with elevated cortisol precisely when you're trying to sleep, which suppresses melatonin and keeps you in a lighter sleep stage.

Cortisol also influences sleep architecture directly. It promotes wakefulness through adrenergic signalling and reduces slow-wave sleep duration. If your cortisol is high at bedtime, you'll spend more time in stage 1 and 2 sleep and less in the deep, restorative stages where growth hormone is released and memory consolidation happens.

The reverse pathway is equally important. Law et al. (2024) showed that poor sleep quality is associated with heightened cortisol reactivity — meaning your cortisol spikes higher and faster in response to acute stressors. You become more physiologically reactive to the same stressor. This is why chronic sleep deprivation feels like it makes you more anxious or irritable; your stress response system is literally amplified.

Sleep timing and cortisol: the circadian mismatch problem

Here's where many people get stuck. You can have adequate sleep duration but still have dysregulated cortisol if your sleep timing is chaotic.

Cho et al. (2024) measured salivary cortisol and melatonin in older adults with dementia and found that those with irregular sleep-wake patterns had flattened cortisol curves — meaning cortisol didn't decline properly at night even though total sleep time was adequate. Their melatonin rhythm was also blunted. The takeaway: you need both adequate sleep and consistent timing.

This is why shift workers and frequent travellers often report feeling unwell despite sleeping enough. Their cortisol and melatonin rhythms are misaligned with their actual sleep window. No supplement can fully override a circadian mismatch, though some ingredients studied for sleep quality may help you adapt more quickly.

What the supplement evidence actually supports

I'm going to be direct here: the evidence for supplements that specifically lower cortisol is weak in humans. Most of the research is in vitro or animal models.

Magnesium and potassium have better human data. Khalid et al. (2024) found that combined magnesium and potassium supplementation improved insomnia severity and normalised sleep hormones in patients with diabetes. But this was a small trial (n=60) and the effect on cortisol specifically wasn't the primary outcome.

Glycine — which is in the KōJō Daily Formula at 2000mg — has preliminary research suggesting it may improve sleep latency and subjective sleep quality. The mechanism appears to involve a drop in core body temperature, which facilitates sleep onset. However, research suggests glycine's direct effect on cortisol levels is limited. What I can say is that if glycine helps you sleep better, and better sleep normalises cortisol, then the benefit is indirect rather than direct.

Taurine (also 2000mg in the Daily Formula) has been studied for sleep quality in animal models, but large-scale human trials are absent. The research is ongoing. I include it because the mechanistic data is interesting — research suggests taurine may modulate GABA signalling — but I'm honest that human RCT evidence showing it improves sleep or cortisol levels is not yet available.

Aged Garlic Extract, Olive Leaf Extract, Grape Seed Extract, and Pine Bark Extract all have antioxidant properties and some in vitro research suggesting they may influence stress pathways. But human trials specifically examining their effect on cortisol levels and sleep are sparse. Research suggests these extracts warrant further investigation, but claiming they directly affect cortisol or sleep would overstate the current evidence base.

The role of consistency and sleep hygiene

If I'm honest about what actually moves the needle on cortisol and sleep, it's not primarily supplements. It's consistency.

Going to bed at the same time each night, waking at the same time each morning, getting daylight exposure in the first hour after waking, and avoiding bright light two hours before bed — these are the interventions with the strongest evidence. They're also free and non-negotiable.

Supplements can support this foundation. But they can't replace it. I've seen people take every sleep supplement on the market whilst maintaining chaotic sleep timing, and they wonder why nothing works. The circadian rhythm doesn't care about your supplement stack if you're fighting it every day.

Stress reduction and the cortisol-sleep loop

Earlier I mentioned that Sladek et al. (2021) found daily rumination predicted elevated cortisol. This matters because it means your thoughts and emotional patterns are actively dysregulating your cortisol rhythm, regardless of what you're taking.

Interventions that reduce rumination — cognitive behavioural therapy for insomnia (CBT-I), mindfulness, journaling — have strong evidence for improving both sleep and cortisol profiles. These should be your first-line interventions. Supplements are adjuncts, not replacements.

That said, if poor sleep is keeping you in a cycle of rumination (tired → more anxious → more rumination → worse sleep), then improving sleep quality with any evidence-based tool — supplement or otherwise — can break the cycle. The direction of causality isn't always clear, but the cycle is real.

Age, cortisol, and sleep architecture changes

Cortisol-sleep dysregulation isn't uniform across the lifespan. Older adults show different patterns than younger adults.

Morgan et al. (2017) found that whilst the cortisol-sleep relationship held across age groups, older adults with short sleep duration had more pronounced elevations in cortisol, and their sleep architecture showed reduced slow-wave sleep. This means the consequences of poor sleep — in terms of cortisol dysregulation and recovery sleep — are more severe in older age.

If you're over 50 and struggling with sleep quality or cortisol levels, this is worth knowing. Your circadian rhythm becomes less flexible with age, so consistency in sleep timing becomes even more important. Research suggests supplements that support sleep continuity, such as glycine or magnesium, may have more noticeable effects in older populations, though the evidence remains limited.

Clinical populations: opioid use disorder and trauma recovery

Two specific populations show interesting cortisol-sleep patterns worth understanding.

Mukherjee et al. (2021) examined cortisol levels and sleep in patients with prescription opioid use disorder during long-term residential treatment. They found that as sleep improved during treatment, cortisol levels normalised — but this took weeks, not days. The reregulation was gradual. This tells me that if your cortisol-sleep dysregulation is rooted in a deeper condition (addiction, trauma, chronic stress), supplements alone won't address the underlying issue. You need the underlying condition addressed first.

Similarly, Daneva et al. (2023) measured salivary cortisol in mild traumatic brain injury patients and found their cortisol levels and physiological parameters fluctuated more than controls, and their sleep was more fragmented. Trauma — whether psychological or neurological — dysregulates cortisol in ways that require professional intervention.

If you're in either category, supplements may help with sleep quality, but they're not a substitute for appropriate clinical care.

Frequently asked questions

Can you take a supplement to lower cortisol directly?

The human evidence is weak. Most "cortisol-lowering" supplements are backed by in vitro or animal data. The strongest indirect approach is improving sleep quality, since Morgan et al. (2017) showed better sleep is associated with lower daytime cortisol. Sleep consistency matters more than any single ingredient.

Does high cortisol always cause insomnia?

Not always, but elevated cortisol at night strongly predicts sleep fragmentation. Wright et al. (2016) found that circadian misalignment and sleep deprivation together create a feedback loop where cortisol stays elevated and sleep worsens. Individual sensitivity varies.

Is glycine effective for sleep?

Research suggests glycine may reduce sleep latency and improve subjective sleep quality through a core body temperature mechanism. The evidence is preliminary and effect sizes are modest. It's not a sedative — it works best when combined with consistent sleep timing and good sleep hygiene.

How long does it take for cortisol to normalise after improving sleep?

Mukherjee et al. (2021) found cortisol reregulation took weeks, not days, even in a controlled treatment setting. If your sleep has been poor for months, expect gradual improvement over weeks to months. Consistency matters more than speed.

Does shift work permanently damage cortisol rhythm?

No, but it's challenging. Cho et al. (2024) showed that irregular sleep-wake patterns flatten cortisol curves. If you return to consistent timing, your rhythm can recover — but the recovery takes time. Light exposure timing and sleep consistency are your best tools.

Should I test my cortisol levels?

If you have symptoms of sleep disruption or suspect a sleep disorder, a healthcare provider can order a salivary cortisol test (usually four samples across the day). This shows your rhythm. However, a single cortisol test isn't diagnostic — context matters.

My honest take

I've read through the cortisol-sleep literature more times than I'd like to admit. Here's what I actually believe, stripped of marketing language.

The cortisol-sleep relationship is real and bidirectional. If your sleep is poor, your cortisol rhythm will suffer. If your cortisol is dysregulated, your sleep will fragment. The evidence on this is solid.

What I'm less convinced about is that supplements are the primary lever for fixing this. The strongest interventions are behavioural: consistent sleep timing, morning light exposure, stress management, and addressing underlying conditions like trauma or chronic illness. These move the needle far more than any ingredient.

That said, I do think supplements can help. If glycine or magnesium helps you fall asleep more easily, and that leads to better sleep consistency, which then normalises your cortisol, then the supplement has value — not because it directly affects cortisol, but because it removed a friction point in your sleep routine. The value is real, just indirect.
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