A circadian rhythm is your body’s roughly 24-hour cycle of physical, mental and behavioral changes. Internal clocks run it, and light keeps it in step with the day. A master clock in the brain, the suprachiasmatic nucleus (SCN), reads the light reaching your eyes and tells the brain to make melatonin in the evening. So you feel sleepy at night and alert by day (NIGMS; NHLBI).
Want to see where your own clock sits? The circadian rhythm calculator uses your sleep times on work days and free days. It estimates your chronotype, your social jetlag and the sleep window your body clock prefers. This page explains the biology behind those numbers.
What Is A Circadian Rhythm?
Circadian rhythms are the physical, mental and behavioral changes an organism goes through over a 24-hour cycle. Most living things have them, including animals, plants and microorganisms. In people, nearly every tissue and organ has its own rhythm. Together they are tuned to the daily cycle of day and night (NIGMS).
The best-known rhythm is the sleep-wake cycle, but it is not the only one. Rhythms also shape hormone release, appetite, digestion and body temperature (NIGMS). They affect metabolism (NINDS), the immune system, thinking and the response to stress (Sleep Foundation). NHLBI says the rhythm touches every cell, tissue and organ in your body (NHLBI).
Where Is Your Body Clock? The SCN And Peripheral Clocks
A biological clock is made of proteins encoded by thousands of genes. They switch on and off in a set order (NIGMS). You have many such clocks, and one master clock coordinates them. In humans it sits in the hypothalamus, in a group of nerve cells called the suprachiasmatic nucleus. The SCN gets light information directly from the eyes (NINDS).
The other clocks, in organs and glands, are called peripheral clocks. They keep their own time. The SCN syncs them so the whole body runs on a shared schedule (NIGMS; Sleep Foundation). People with SCN damage can sleep erratically through the day. They cannot match their sleep-wake cycle to the light-dark cycle (NINDS).
How does a single cell keep time? The 2017 Nobel Prize went to three researchers who worked it out in fruit flies. A protein called PER builds up overnight, guarded by a partner protein called TIM. Then it switches off its own production. At daybreak the complexes break down and the cycle restarts. This self-limiting loop gives a cell its roughly daily rhythm (NIGMS).
Why The Clock Needs Resetting Every Day
Your internal clock does not run at exactly 24 hours. In carefully controlled lighting, the human pacemaker averaged 24.18 hours in both young and older adults (Czeisler and colleagues, 1999). Most people’s natural cycle runs a little long (NHLBI).
So the master clock must adjust by roughly 12 to 18 minutes every day to match the planet’s rotation (Sleep Foundation). It uses signals from the outside world.
Zeitgebers: What Sets Your Clock
Outside signals that set the clock are called zeitgebers, German for “timekeepers”. Light and darkness are the most important and most powerful (Sleep Foundation). NHLBI calls light the strongest signal for resetting the sleep-wake cycle (NHLBI). Special cells in the retina tell the brain whether it is day or night (NINDS).
Light is not the only cue. Food, stress, physical activity, social setting and temperature also affect circadian rhythms (NIGMS). Your body tries to line up sleep with outdoor light and dark, meal times and activity (NHLBI). Social contact and daily routines count too (Sleep Foundation).
| Zeitgeber | Role | What it means day to day |
|---|---|---|
| Light and darkness | The strongest cue | Daylight, especially in the morning, anchors the clock; bright light late in the evening pushes it later |
| Meal timing | Also a cue | Regular meal times help, especially for shift workers and people who sleep at irregular times |
| Physical activity | Also a cue | Regular daytime activity supports the rhythm; exercise close to bedtime may make sleep harder |
| Social contact and routine | Also a cue | Fixed daily routines, work and school times shape when you sleep |
Sources: Sleep Foundation, NIGMS, NHLBI.
Your Body Clock Across The Day: Melatonin, Cortisol And Temperature
The SCN controls melatonin based on how much light the eyes receive. In the evening, the master clock tells the brain to make more melatonin, which causes sleepiness (NIGMS). The pineal gland makes it. Its peaks and valleys help match your rhythm to the outside light and dark (NINDS).
| Part of the day | What the clock does |
|---|---|
| Evening, as it gets dark | Melatonin starts to rise and core body temperature drops, both of which reduce alertness |
| Night | Melatonin keeps rising and peaks in the early morning hours |
| Morning, as light increases | Melatonin production stops, cortisol is released to prepare the body to wake, and body temperature rises |
| Daytime | Melatonin is very low and the clock promotes alertness |
Sources: NHLBI, Sleep Foundation, AASM Sleep Education.
Exact clock times differ from person to person. That is the point of chronotypes, covered below. Bright artificial light late in the evening can stop the brain releasing melatonin. TVs, phones and very bright alarm clocks all count (NHLBI). To check your evening light, try the blue light exposure calculator.
How The Clock Works With Sleep Pressure
The clock decides when you are ready to sleep, but it does not work alone. A second system, sleep pressure, builds the longer you are awake. Part of the cause is a compound called adenosine, which rises in the brain. Caffeine works by blocking adenosine (NHLBI).
The circadian rhythm and this sleep drive together decide when you are awake and asleep (NINDS). In the evening, rising melatonin and falling body temperature combine with sleep drive to make you fall asleep (Sleep Foundation).
Scientists call this the two-process model. We explain it step by step, with a side-by-side table, in how sleep works. To see how your last coffee overlaps your bedtime, use the caffeine impact on sleep calculator.
Chronotypes: Why Your Clock Is Not The Same As Everyone Else’s
Some people naturally wake early and some stay up late (NHLBI). Your preferred timing is your chronotype. It is partly set by your genes, and some people adjust their rhythm to their surroundings more easily than others (NHLBI).
Timing also shifts across life. Rhythms begin developing after birth, and babies start to sync with the light-dark cycle around 2 to 4 months. During puberty the rhythm shifts later, so teens feel sleepy later and want to wake later (Sleep Foundation).
From about age 60 to 65 the rhythm moves earlier again, so many older adults get sleepy earlier in the evening (Sleep Foundation). They usually sleep less and wake earlier (NHLBI).
For how researchers measure chronotype, what social jetlag is, and why animal types are not research categories, see chronotypes explained.
What Disrupts Your Circadian Rhythm?
Rhythms can fall out of sync with the outside world because of factors in the body or the environment (NIGMS). The common ones:
- Shift work. Working when your body expects sleep sets your clock against your schedule. Melatonin may rise when you need to be alert. Daylight on the way home can block it when you need to sleep (AASM). Night owls may adjust more easily to evening shifts, and morning types to early shifts (AASM).
- Jet lag. After you fly across at least two time zones, your sleep-wake cycle may stay on home time. Flying east is usually harder than flying west. Full adjustment is estimated at about one day per time zone (AASM). The jet lag recovery calculator estimates how many days you need and when to seek or avoid light on day one.
- Light at the wrong time. Light from devices at night can confuse biological clocks (NIGMS). Too little daylight during the day is a risk factor too (NHLBI).
- Irregular schedules. Staying up late, sleeping in at weekends and often changing bedtime can confuse the body clock (Sleep Foundation). If your plans keep slipping, see why your sleep schedule keeps failing.
- Daylight saving time. The sudden one-hour change disrupts sleep. Its effects on sleep-wake patterns last about 5 to 7 days. They are probably worst for people who start the change short on sleep (AASM). The spring change is usually the harder one (Sleep Foundation).
- Caffeine, alcohol and other habits. Alcohol use, chronic caffeine use and frequent air travel all raise the risk of circadian rhythm problems (NHLBI).
- Health conditions. Variants of certain genes and neurological diseases such as Alzheimer’s disease can also disrupt rhythms (NIGMS).
Shift workers can use the shift work sleep calculator to plan sleep, naps and caffeine around shifts. Our guide on how to sleep after a night shift covers the day after.
In the short term, an out-of-sync clock can cause drowsiness, poor coordination and trouble with learning and focus. Long-term sleep loss and constantly shifting rhythms can raise the risk of obesity, diabetes, mood disorders, heart and blood pressure problems and cancer (NIGMS).
How To Reset Your Circadian Rhythm
Resetting means moving your clock back in line with the schedule you need. NHLBI lists these steps for realigning the sleep-wake cycle (NHLBI):
- Manage light first. Get more daylight during the day and less screen light at night. Dim lighting for a while before bed may help.
- Keep regular sleep and wake times and a relaxing bedtime routine in a cool, quiet room.
- Eat at regular times, especially if you work shifts or sleep at irregular hours.
- Be active during the day and avoid exercise close to bedtime.
- Avoid daytime naps, especially in the afternoon. Shift workers are the exception: a short nap before a shift may help.
- Limit caffeine, alcohol and nicotine, especially near bedtime. For circadian rhythm disorders, a doctor may suggest no caffeine within eight hours of your desired bedtime.
Light timing matters because light can push the clock either way. Bright light soon after waking moves sleep and wake times earlier. Bright light in the late afternoon or early evening moves them later (NHLBI).
Change in small steps, not all at once. Before daylight saving time, for example, the AASM suggests shifting sleep and wake times gradually, starting 2 to 3 days ahead (AASM).
Need a plan with dates? The sleep schedule reset calculator turns your current and target sleep times into a day-by-day plan for bedtime, alarm, light and last caffeine.
When Is It A Circadian Rhythm Disorder?
Circadian rhythm disorders, also called sleep-wake cycle disorders, happen when your internal clock is out of sync with your environment. The mismatch gets in the way of daily life. It can be short-term, from habits, work or travel. It can be long-term, from aging, genes or a medical condition. Symptoms include extreme daytime sleepiness, lower alertness and problems with memory and decisions (NHLBI).
The most common in young people is delayed sleep-wake phase disorder (DSP). Sleep is pushed two or more hours later than social norms. You cannot fall asleep until late and struggle to get up for work or school. On your own schedule, though, you sleep a normal amount and quality, just later. It can look like insomnia.
About 7 to 16 percent of teens and young adults may have it. Evening types are more likely to develop it. About 40 percent of people with DSP have a family history of it (AASM Sleep Education). The opposite pattern is advanced sleep-wake phase disorder, with very early sleep and waking. It is more common in older adults (NHLBI).
| Being a night owl | Delayed sleep-wake phase disorder | |
|---|---|---|
| Sleep timing | Prefers later bed and wake times | Sleep delayed two or more hours from social norms, stable for at least 7 days |
| Fits daily life? | Can usually keep required hours, though they may not feel at their best until later in the day | Interferes with work, school or social demands |
| Daytime effects | May feel tired on early work or school days | Can impair alertness and performance; excessive sleepiness and fatigue |
| Who to see | Self-help steps above | A sleep doctor, ideally with a sleep diary |
Sources: AASM Sleep Education, Sleep Foundation.
Treatment depends on the type. Common options are lifestyle changes, bright light therapy and melatonin, often combined (NHLBI). The AASM’s clinical practice guideline supports strategically timed melatonin for DSP in adults. It supports light therapy, with or without behavioral treatment, for children and teens with DSP and for adults with the advanced form (Auger and colleagues, AASM 2015).
Set melatonin timing and amount with a sleep doctor (AASM). Light therapy can cause side effects such as eye strain and headaches. Ask your doctor first if you have an eye condition or take medicines that make you sensitive to light (NHLBI).
When To See A Doctor
- Your sleep timing is out of step with work, school or family life for weeks and the steps above have not helped (NHLBI).
- You are very sleepy during the day or at work, or you have nearly fallen asleep while driving. Untreated circadian rhythm disorders can raise the risk of workplace and road accidents (NHLBI).
- Shift work leaves you struggling to sleep or severely tired for a month or more, and it affects your work, family or social life (AASM).
- You travel often and keep struggling with jet lag, or you suspect another sleep disorder such as sleep apnea (AASM).
A doctor may ask you to keep a sleep diary for a couple of weeks to show your pattern of sleep times (AASM). If trouble sleeping is the main problem rather than timing, start with our insomnia guide.
Circadian Rhythm FAQs
What Is A Circadian Rhythm In Simple Terms?
It is your body’s internal 24-hour clock. It times sleepiness, alertness, hormones, appetite and body temperature.
What Controls Your Circadian Rhythm?
The master clock in the brain, the suprachiasmatic nucleus. It reads light from the eyes and keeps the smaller organ clocks in sync.
Is The Body Clock Exactly 24 Hours?
No. For most people it runs slightly longer, so daylight and daily routines nudge it back into line each day.
How Long Does It Take To Reset Your Circadian Rhythm?
It depends on how far the clock has to move. Jet lag and daylight saving changes take days, not hours, as the list above shows.
Can You Reset Your Circadian Rhythm In One Day?
Not fully. Morning light and a fixed wake time start the shift at once. The clock moves in steps, so plan for several days.
Is A Circadian Rhythm Calculator The Same As A Chronotype Test?
Close. Our circadian rhythm calculator estimates your chronotype from your real sleep times, not from preference questions.