Circadian Rhythm Calculator

About it

What is a circadian rhythm?

It's your body's internal clock: a cycle of roughly 24 hours that runs in the background and tells you when to feel awake and when to feel sleepy. The name comes from the Latin circa diem, "about a day".

The master clock is a group of about 20,000 nerve cells in the hypothalamus called the suprachiasmatic nucleus (SCN). It sets itself mostly by light reaching your eyes. Morning light stops the release of melatonin, the sleep hormone, so you feel alert. As it gets dark, melatonin rises and you start to feel sleepy. The same clock also keeps time for hormones, body temperature, digestion and metabolism.

Jet lag, shift work, bright screens late at night and an irregular sleep schedule can pull the clock out of step with your day. That can make it harder to sleep and affect how you feel, which is why a steady routine and daylight in the morning help.

Sources: Mayo Clinic, What are circadian rhythms?, NIGMS (National Institutes of Health), Circadian Rhythms, Cleveland Clinic, Circadian Rhythm.

What it does

Tell it when you need to wake up, or when you want to go to bed, and it suggests times that line up with whole sleep cycles. Pick one and it lays out your day: when focus peaks, when the afternoon dip hits, the last sensible coffee and when to start winding down.

Why 90 minutes

Sleep runs in cycles of light, deep and REM sleep that last about 90 minutes on average. Waking at the end of a cycle, when sleep is lightest, usually feels easier than being pulled out of deep sleep. Most adults need 7 to 9 hours, which is 5 or 6 cycles.

The day

Your body clock runs on a roughly 24-hour rhythm set mostly by light. For someone who sleeps at night, alertness rises through the morning, dips in the early afternoon, picks up again in the late afternoon and falls as melatonin rises in the evening. The times shown are typical offsets from when you wake up. The research behind each one is listed under Sources.

Disclaimer

General guidance, not medical advice. Real cycles run 70 to 120 minutes and change through the night, and everyone's clock is a little different. If you struggle with sleep, talk to a doctor.

Sources

Sleep cycles of about 90 minutes, 4 to 6 a night: Patel et al., Physiology, Sleep Stages, StatPearls.

7 or more hours for adults: Watson et al., AASM and Sleep Research Society consensus, 2015.

Grogginess after waking (sleep inertia): Hilditch and McHill, Sleep inertia: current insights, 2019.

Light sets the body clock: Blume, Garbazza and Spitschan, Effects of light on human circadian rhythms, sleep and mood, 2019.

The afternoon dip: Monk, The post-lunch dip in performance, 2005.

Short naps: Tietzel and Lack, The recuperative value of brief and ultra-brief naps, 2002.

Caffeine and sleep: Drake et al., caffeine 0, 3 or 6 hours before bed, 2013, and Gardiner et al., meta-analysis of 24 studies, 2023, which puts a cup of coffee at least 8.8 hours before bed.

Physical performance peaks in the late afternoon: Drust et al., Circadian rhythms in sports performance, 2005.

Melatonin rises about 2 hours before bedtime: Burgess et al., dim light melatonin onset and sleep, 2003.

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