Cortisol and exercise: the daily rhythm, hard sessions, caffeine and sleep

A hormone with a daily shape

Cortisol is often called the stress hormone, which makes it sound like something to suppress. In fact it follows a predictable daily pattern in everyone. The marked rise after waking is called the cortisol awakening response. From that morning peak, levels decline through the afternoon and evening.

The shape of that curve seems to matter. A 2017 meta-analysis of 80 studies by Adam and colleagues found that a flatter slope from morning to evening was associated with poorer mental and physical health. The average effect was small, and the authors discuss whether a flat curve is a cause or simply a marker of other problems. Either way, the useful goal is not low cortisol. It is a clear rhythm: high in the morning, low at night.

What a training session does to it

Intensity decides. Hill's study in 12 trained men showed no meaningful rise at an easy pace and large rises at moderate-to-hard and hard efforts. After correcting for changes in blood volume, the easy session actually lowered cortisol. Whatever the size, the rise after a hard session is short-lived and part of a normal response to effort.

The longer-term picture is reassuring. A systematic review by Mücke and colleagues found that fitter, more active people showed smaller cortisol responses to a standard laboratory stress test in 7 of 12 studies. For the full distinction between the acute spike and the adaptation to regular training, see exercise and hormones. For what training does to the experience of pressure at work, see exercise and stress.

Caffeine: a real but limited effect

In a study by Lovallo and colleagues, caffeine raised cortisol across the waking hours. In people who drank coffee every day, the response was reduced but not eliminated, and levels returned to normal in the evening. So the morning espresso does nudge cortisol upwards, on top of the natural morning rise.

The more important effect of caffeine is on sleep, and sleep in turn shapes cortisol. Drake and colleagues found that 400 milligrams of caffeine, about four standard cups, still disrupted sleep when taken six hours before bed. A 2023 meta-analysis by Gardiner and colleagues went further. To avoid losing total sleep, a coffee of about 107 milligrams should be drunk at least 8.8 hours before bed, and a typical pre-workout supplement of 217.5 milligrams at least 13.2 hours before. For a bedtime of 23:00, that means the last coffee by about 14:00. The caffeine guide covers doses in more detail.

Short nights shift the evening curve

Leproult and colleagues kept volunteers awake for part or all of a night and measured cortisol the following evening. Between 18:00 and 23:00, it was 37% higher after partial sleep loss and 45% higher after a full night without sleep. The quiet period of the night, when cortisol normally falls to its lowest, started at least an hour later.

That creates a loop most frequent travellers will recognise. A short night raises evening cortisol, which makes the next night harder. Sleep is the lever here, and the sleep for executives guide is the place to start.

Evening training and bedtime

Many executives can only train after the last meeting. The evidence says this is generally fine. A 2019 meta-analysis by Stutz and colleagues found that evening exercise did not harm sleep, and if anything the opposite, with one exception: vigorous exercise ending an hour or less before bed might delay sleep and shorten it. A 2021 review by Frimpong and colleagues found that hard sessions ending two to four hours before bed did not disrupt sleep in healthy good sleepers.

So keep a buffer of at least an hour after a hard session, and two or more if you sleep poorly. An easy session, such as a walk or mobility work, can end closer to bedtime. In Dubai's summer, when late-evening sessions are common because of the heat, that buffer is the main thing to protect. Best time to workout helps choose the window.

The cortisol belly claim

You will read that stress hormones cause belly fat, often sold with a supplement. It rests on limited evidence. The study usually cited, by Epel and colleagues in 2000, found that women with more central fat secreted more cortisol during repeated laboratory stress. It was cross-sectional, in 59 premenopausal women, so it shows an association and cannot say which came first. Belly fat responds to overall energy balance, activity and sleep, not to a cortisol-lowering product.

The same caution applies to home measurement: no saliva kit or wearable score can diagnose a cortisol problem.

When cortisol is a medical question

Cushing's syndrome, caused by too much cortisol, is rare. The NHS notes that its most common cause is taking high doses of steroid medicines for a long time. Signs include rapid weight gain on the face or trunk, purple stretch marks, easy bruising and muscle weakness.

If you notice those signs, see a doctor. Never stop prescribed steroids on your own. Persistent fatigue, low mood or unexplained weight gain also deserve a medical opinion rather than a home test.

A day that respects the rhythm

1. Morning: the natural cortisol rise comes on its own after waking; an early session of any intensity fits it.
2. Midday: a brisk walk or a moderate session; the walking meetings guide shows how to fit one in.
3. Last coffee: about nine hours before your usual bedtime.
4. Hard training: finished at least an hour before bed, ideally two.
5. Evening: easy movement, dim light and a regular bedtime, even after a late dinner.

The calculator checks your own timings against those intervals. If you want a coach to place your sessions around your diary, you can join the Club.