What is the circadian rhythm in simple terms?

It is your body’s daily cycle: physical, mental and behavioral changes that repeat over about 24 hours, such as sleeping and waking. A master clock in your brain keeps it running (NIGMS).

Is the circadian rhythm the same as the body clock?

Not quite. The circadian rhythm is the cycle itself. The biological clock, often called the body clock, is the system that keeps time and controls that cycle (NIGMS).

How long is a day on your body clock?

Slightly longer than 24 hours on average: about 24.18 hours in a carefully controlled lab study (Czeisler et al., 1999). Light and dark set it again each day.

Can you change your circadian rhythm?

To a degree. Light in the morning can move the clock earlier and light in the evening can move it later (Spitschan et al., 2025). How much yours moves differs from person to person, so give a new routine several days.

Guide · Body clock

Circadian rhythm explained: how your body clock works

Your body runs on a cycle of roughly one day. A clock in your brain keeps that cycle going, and on its own it runs a little longer than a day. Light and dark set it again each day, which is why the light you get matters to it.

LightOtter editorial team · reviewed against the sources below · 14 October 2026 · Who is behind it

The short answer

What is the circadian rhythm?

The circadian rhythm is your body’s cycle of roughly 24 hours, kept by a master clock in your brain. Left to itself, that clock runs a little long: in a carefully controlled lab study, its period averaged 24.18 hours (Czeisler et al., 1999). Light and dark reset it every day, so it stays in step with the day outside.

What does circadian mean?

The word comes from Latin: circa means “around” and dies means “day” (Nobel Prize press release, 2017). A circadian rhythm covers physical, mental and behavioral changes that run over a 24-hour cycle, such as when you sleep and when you are awake (NIGMS).

Humans are not special here. Animals, plants and microorganisms have circadian rhythms too (NIGMS). In the 18th century, the astronomer Jean Jacques d’Ortous de Mairan put a mimosa plant in constant darkness. Its leaves kept opening and closing on their daily schedule anyway (Nobel Prize press release, 2017).

Where is the body clock located?

The system that keeps time and controls circadian rhythms is called a biological clock. Your body has many of them, and a master clock coordinates the rest (NIGMS).

In humans, the master clock is a large group of nerve cells in the brain called the suprachiasmatic nucleus, or SCN. It keeps the rhythms of different organs and tissues in sync. Nearly every tissue and organ has its own circadian rhythm (NIGMS).

How long is the circadian rhythm?

On average, a little longer than 24 hours. Older studies reported about 25 hours, but the people in them were exposed to enough light to skew the measurement (Czeisler et al., 1999). Later studies controlled the light carefully:

24.18 hours: the average period of the human body clock, the same in younger and older adults (Czeisler et al., 1999).
24 hours 9 minutes: the average across 157 people aged 18 to 74, measured during a month-long stay in the lab (Duffy et al., 2011).
24 hours 5 minutes for women, 24 hours 11 minutes for men: the averages in that same study (Duffy et al., 2011).
35% of women and 14% of men: the share whose internal day was shorter than 24 hours (Duffy et al., 2011).

Why does a few minutes a day matter?

A few minutes sounds like nothing. Without a daily reset, though, they would add up, and your inner day would slowly drift away from the real one. What keeps it in line are outside cues, and light and dark have the biggest influence on circadian rhythms (NIGMS).

How does the body clock work inside your cells?

In the 1970s, Seymour Benzer and Ronald Konopka showed in fruit flies that mutations in an unknown gene disrupted the flies’ clock. They named the gene period (Nobel Prize press release, 2017).

In 1984, Jeffrey Hall and Michael Rosbash in Boston and Michael Young in New York isolated it. Hall and Rosbash then found that PER, the protein made from period, builds up during the night and is broken down during the day (Nobel Prize press release, 2017).

At night, a second protein called TIM binds to PER. Together they enter the cell nucleus and stop the cell from making more PER. As day breaks, the pairs fall apart, the block lifts and the cycle starts again. PER controls its own production through a feedback loop (NIGMS).

Young found the gene for TIM in 1994. He also found doubletime, a gene that delays the buildup of PER, which helped explain how the cycle is tuned to stay close to 24 hours (Nobel Prize press release, 2017).

Biological clocks in the cells of other multicellular organisms, humans included, work by the same principles (Nobel Prize press release, 2017).

Who won the Nobel Prize for the circadian rhythm?

Jeffrey C. Hall, Michael Rosbash and Michael W. Young shared the 2017 Nobel Prize in Physiology or Medicine “for their discoveries of molecular mechanisms controlling the circadian rhythm” (Nobel Prize press release, 2017).

How does light affect the circadian rhythm?

An expert consensus puts it simply: light in the morning can move the clock earlier, and light in the evening can move it later (Spitschan et al., 2025).

An international expert panel has turned this into numbers. It recommends at least 250 lux melanopic EDI at the eye during the day, at most 10 lux in the evening starting at least 3 hours before bedtime, and a sleep environment as dark as possible (Brown et al., 2022). Melanopic EDI is a standard measure of how strongly light acts on the light-sensitive cells in the eye that relay these effects (Brown et al., 2022).

Natural light alone can line the clock up with the sun. In a camping study, after people lived with only natural light, the start of their internal night came at sunset (Wright et al., 2013).

How long should you stay out this morning? The morning light calculator estimates it for your weather.

What else sets your body clock besides light?

Researchers call the signals that set the clock zeitgebers, or time cues. Bright light is regarded as the main one (Youngstedt et al., 2019). Food intake, physical activity, social environment and temperature also affect circadian rhythms (NIGMS).

Meal times: in a small lab study, ten healthy young men ate all their meals five hours later for six days. Markers of the master clock did not shift, but the blood sugar rhythm moved more than five hours later (Wehrens et al., 2017). Meal timing acted on clocks in the body, not on the master clock.

Exercise: 51 older and 48 younger adults did one hour of moderate treadmill exercise on three days in a row. Exercise at 7 am or between 1 and 4 pm moved the clock earlier, and exercise between 7 and 10 pm moved it later. The shifts were about as large as expected from bright light of the same length (Youngstedt et al., 2019). Whether an easy walk does the same was not part of the study.

Early birds, night owls and everyone in between

Body clocks do not all run at the same time of day. Researchers call how early or late yours runs your chronotype. In the camping study, later chronotypes shifted earlier the most once they had only natural light, which brought them closer to the earlier types (Wright et al., 2013).

Early bird or night owl? Find out with the 2-minute chronotype quiz.

What throws your circadian rhythm off?

Circadian rhythms can fall out of sync with the world outside because of factors in the body or the environment (NIGMS). Common ones:

Jet lag and shift work, which change the normal sleep-wake cycle (NIGMS).
Light from electronic devices at night, which can confuse biological clocks (NIGMS).
Less sunlight during the day and more light after sunset: people living with electric light had a later body clock than when camping with natural light only (Wright et al., 2013).
Variants of certain genes that affect the proteins controlling the clock (NIGMS).

What happens to your body clock on night shifts?

Shift work forces abrupt changes in when people sleep and when they see light. The body clock then falls out of step with the light-dark cycle outside (Boivin et al., 2022).

It is slow to switch from day to night: in simulated night shifts and in studies of shift workers, it barely shifted even over several days. The clocks inside the body also drift apart. After several days of living at night, most rhythmic genes stayed set to a day schedule, while many metabolites moved by several hours (Boivin et al., 2022).

The tips on this page assume a regular day. If you work shifts, ask an occupational medicine doctor how to time your light and sleep.

How to keep your body clock in step

These are small habits you can repeat every day, which makes them easier to keep than one big change.

Go outside soon after you get up. A recent policy brief advises daylight each day, especially within the first few hours after waking (Piper et al., 2026). The same brief says the evidence does not justify one fixed number of minutes for everyone.
LightOtter uses its own rule of thumb, an estimate rather than a study result: about 10 minutes on a clear morning, 15 when it is partly cloudy, 20 to 25 when it is overcast and up to 30 with a low sun. Never look directly at the sun.
Dim your evenings: warmer, lower lamps and darker screens in the last hours before bed.
Keep a steady wake-up time, within about an hour on weekends too.

Want to shift your clock earlier? Follow the 7-day plan to reset your circadian rhythm.

What this is not

This page explains how the body clock works. It is not medical advice, and light is not a substitute for enough sleep. If you feel unusually tired for weeks or have long-lasting trouble sleeping, talk to a doctor.