What Is the Circadian Rhythm? How Light and Timing Set Your Sleep

In the standard model of sleep regulation, when you feel sleepy depends on two things: how long you have been awake, and what time your body’s clock says it is. Our guide to sleep pressure covers the first. This guide covers the second, the circadian rhythm. It explains what it is, how light moves it, what melatonin does in it, and what the research says about supplements and light aimed at it.

It is not medical advice. If sleep trouble is persistent, talk to a healthcare provider.

The Short Answer

The circadian rhythm is the set of changes your body goes through over about 24 hours. The U.S. National Institute of General Medical Sciences says light and dark have the biggest influence on it, and that food, stress, physical activity, social environment and temperature also affect it.
The clock does not run at exactly 24 hours. In a 1999 laboratory study, the intrinsic period of the human clock averaged 24.18 hours, and light is the biggest cue that keeps it in step with the day.
When you get light matters. In a laboratory study, one hour of bright light shifted the clock by an amount that depended on the time of exposure, and dim light shifted nothing measurable.
Melatonin is the hormone the master clock controls. The American Academy of Sleep Medicine’s guideline on circadian rhythm disorders endorses melatonin given at set times in specific disorders. That guideline does not cover general insomnia.
The practical light levels come from an expert consensus for healthy adults, built on laboratory work. Its authors say some questions remain open.

What the Circadian Rhythm Is

The National Institute of General Medical Sciences describes circadian rhythms as the physical, mental and behavioral changes an organism goes through over a 24-hour cycle. In humans, nearly every tissue and organ has its own rhythm, and together they are tuned to the daily cycle of day and night. The rhythms influence sleep patterns, hormone release, appetite and digestion, and temperature. The master clock is a group of nerve cells in the brain called the suprachiasmatic nucleus. It controls production of the hormone melatonin based on the amount of light the eyes receive. In the evening, it tells the brain to make more melatonin, which causes sleepiness (NIGMS, content reviewed September 2023).

Its period is not exactly a day. Czeisler and colleagues measured the melatonin, body temperature and cortisol rhythms of healthy young and older adults living in carefully controlled lighting. They found that the intrinsic period of the human circadian pacemaker averages 24.18 hours in both age groups, with a tight distribution. Earlier estimates of about 25 hours or more, they said, came from studies in which light levels were high enough to confound the estimate (Czeisler et al., 1999). Light is the biggest cue that keeps the clock in step with the day.

Light: The Main Reset

Timing. The effect of light on the clock depends on when it arrives. St Hilaire and colleagues exposed 36 people to either one hour of bright white light (about 8,000 lux) or dim light (below 3 lux) at one of 18 points in their circadian cycle. The bright light produced a response curve with a peak-to-trough amplitude of 2.20 hours. The dim light produced no discernible curve (St Hilaire et al., 2012). This was a laboratory study of a single hour of very bright light. It is not a home routine.

Evening light. A 2019 systematic review of 128 articles, mostly on melatonin, found that a two-hour exposure to blue light (460 nm) in the evening suppresses melatonin, and that melatonin recovered within about 15 minutes after the light ended. It also found that light in the evening, at night and in the morning all affected the timing of the melatonin rhythm. The review’s authors point out that most studies were of adults in their twenties or thirties, that 39 of the 128 had ten or fewer participants, and that very few involved children or older adults (Tähkämö et al., 2019).

What experts recommend. A 2022 report in PLOS Biology sets out consensus recommendations from a 2019 workshop of lighting, circadian and sleep experts. It is for healthy adults and it is expert consensus. It uses a measure called melanopic equivalent daylight illuminance (melanopic EDI), which quantifies the influence of light on the light-sensing nerve cells in the retina that drive these effects. Its recommendations are:

Daytime: maximize light exposure, with a melanopic EDI above 250 lux.
The three hours before your usual sleep time: below 10 lux.
The sleep environment: below 1 lux.

Brown and colleagues note that a display in an otherwise dark room can by itself give more than 60 lux. Some laboratory studies have found that light from modern displays can reduce the evening rise in melatonin and increase alertness, and that reducing short-wavelength light from them lessened these effects. They also say there have been no large field studies of how well this works in daily life, and that adjusting a display’s colors cuts its melanopic output by only about half, so such changes are likely to help most when combined with dimming screens and low ambient light. Whether good daytime light protects against the effects of evening light, they add, needs future research (Brown et al., 2022).

Melatonin and the Clock

Melatonin is the hormone the master clock controls according to the light the eyes receive, making more of it in the evening (NIGMS).

The American Academy of Sleep Medicine updated its guideline on the intrinsic circadian rhythm sleep-wake disorders in 2015. Its review of the evidence supported strategically timed melatonin, with second-tier confidence, for delayed sleep-wake phase disorder, for blind adults with non-24-hour sleep-wake rhythm disorder, and for children and adolescents with irregular sleep-wake rhythm disorder and a neurological disorder. It recommended against melatonin for older people with dementia. Light therapy, with or without behavioral measures, was supported for adults with advanced sleep-wake phase disorder, for children and adolescents with delayed sleep-wake phase disorder, and for older people with dementia. For the remaining options and groups, the authors gave no recommendation because the data were insufficient or absent (Auger et al., 2015).

A 2021 systematic review of eight papers looked at bright light and melatonin together. In healthy participants, the combination outperformed either alone in moving the melatonin rhythm earlier, but not in moving it later. In the two studies of people with delayed sleep-wake phase disorder it showed no benefit, while in the two studies of older people with cognitive decline it was more beneficial than a single therapy. The authors say adverse effects of melatonin in older people must be considered (Cheng et al., 2021).

We cover what melatonin does and does not do for sleep, the doses in studies and how accurate the labels have been in our melatonin guide. We cannot tell you when or how much to take. Timing is what these guidelines are about, and it is a decision for a clinician.

People differ in when they sleep. Roenneberg and colleagues define chronotype as individual differences in sleep timing, determined by genetic background, age, sex and environment, including light exposure. They define social jetlag as the discrepancy between the circadian and the social clock, which they say results in chronic sleep loss. In a large epidemiological study they found that social jetlag was associated with a higher body mass index, beyond the effect of sleep duration (Roenneberg et al., 2012). This is an association in observational data. The study does not show that social jetlag causes weight gain.

When the Clock Runs Out of Step

The National Institute of General Medical Sciences lists several things that can put the clock out of step with the outside world: variants in clock genes, neurological diseases such as Alzheimer’s disease, travel across time zones, shift work, and light from electronic devices at night. It says short-term mismatch can bring drowsiness, poor coordination and difficulty with learning and focus (NIGMS).

The AASM guideline covers four named disorders in which the clock is persistently out of step: advanced sleep-wake phase disorder, delayed sleep-wake phase disorder, non-24-hour sleep-wake rhythm disorder and irregular sleep-wake rhythm disorder (Auger et al., 2015). If you can never fall asleep until very late and cannot wake at the time you need to, or your sleep times shift from day to day, that is a reason to talk to a healthcare provider, not to handle it on your own with a supplement.

What We Do Not Know

How well the consensus light levels work in everyday homes. The report says there are no large field studies of some of its suggestions.
Whether daytime light protects against evening light. The consensus report says this needs research.
The right timing and dose of melatonin for you. The guideline’s recommendations are for specific disorders.
How these findings apply to children and older adults. The 2019 review found few studies in them.
Why some people are more sensitive to light than others.

Questions

What is the circadian rhythm?

The physical, mental and behavioral changes the body goes through over a 24-hour cycle, run by a master clock in the brain and shaped mostly by light and dark.

Is the body clock exactly 24 hours?

In a 1999 study under controlled lighting the average was 24.18 hours, and light and other cues keep it in step with the day.

Does light from screens at night affect sleep?

Laboratory studies have found that display light can reduce the evening rise in melatonin and increase alertness. A 2022 expert report suggests keeping the three hours before sleep below 10 lux. It also says there are no large field studies of how much display changes help in daily life.

Does melatonin reset the body clock?

Melatonin is the signal the clock produces in the dark, and the AASM guideline supports timed melatonin in specific circadian disorders. What it says about general sleep problems is covered in our melatonin guide.

Sources Cited in This Guide

National Institute of General Medical Sciences, NIH. “Circadian Rhythms.” Content reviewed September 2023. Read September 19, 2026. NIGMS
Czeisler CA, et al. “Stability, precision, and near-24-hour period of the human circadian pacemaker.” Science. 1999. Read September 19, 2026 (abstract). PMID 10381883
St Hilaire MA, Gooley JJ, Khalsa SB, Kronauer RE, Czeisler CA, Lockley SW. “Human phase response curve to a 1 h pulse of bright white light.” J Physiol. 2012. Read September 19, 2026 (abstract). PMID 22547633
Tähkämö L, Partonen T, Pesonen AK. “Systematic review of light exposure impact on human circadian rhythm.” Chronobiol Int. 2019. Read September 19, 2026 (abstract). PMID 30311830
Brown TM, et al. “Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults.” PLoS Biol. 2022. Read September 19, 2026 (full text). PMID 35298459
Auger RR, et al. “Clinical Practice Guideline for the Treatment of Intrinsic Circadian Rhythm Sleep-Wake Disorders: Advanced Sleep-Wake Phase Disorder (ASWPD), Delayed Sleep-Wake Phase Disorder (DSWPD), Non-24-Hour Sleep-Wake Rhythm Disorder (N24SWD), and Irregular Sleep-Wake Rhythm Disorder (ISWRD). An Update for 2015: An American Academy of Sleep Medicine Clinical Practice Guideline.” J Clin Sleep Med. 2015. Read September 19, 2026 (abstract). PMID 26414986
Cheng DCY, et al. “The efficacy of combined bright light and melatonin therapies on sleep and circadian outcomes: A systematic review.” Sleep Med Rev. 2021. Read September 19, 2026 (abstract). PMID 33962317
Roenneberg T, Allebrandt KV, Merrow M, Vetter C. “Social jetlag and obesity.” Curr Biol. 2012. Read September 19, 2026 (abstract). PMID 22578422

This guide is for information and is not medical advice. Talk to a healthcare provider about medication, pregnancy, medical conditions and persistent sleep problems.