Buhata
Login Start for Free

Home › Life Terms

Wellbeing and habits

Circadian rhythm

The body's roughly 24-hour cycle of physical, mental and behavioral changes, such as sleepiness at night, kept in time by a clock in the brain that follows light.

What It Means

A circadian rhythm is a cycle of physical, mental and behavioral changes that repeats about every 24 hours, such as sleepiness at night and alertness during the day. Humans keep these rhythms in time with a master clock in the brain, a group of nerve cells called the suprachiasmatic nucleus (SCN). Light reaching the eyes is the main cue that sets it. As evening comes, the SCN has the brain make more melatonin, a hormone that brings on sleepiness.

Try It: How Far Do Days Off Move Your Sleep?

Pick your usual bedtime and wake-up time on workdays and on days off. The tool finds the middle of each night and how far apart they sit.

Workdays
Time in bed (hours)7.5Middle of the night

2:45 am

Days off
Time in bed (hours)8.5Middle of the night

4:45 am

Gap between the two middles (hours)2.0

Sleep researchers call this gap social jetlag.

Days off move your sleep later. CDC and AASM both advise one wake-up time for every day, days off included.

How People Say It

  • "My body clock still thinks I'm on vacation."
  • "I'm a night owl, my rhythm just runs late."
  • "Night shifts wreck my circadian rhythm."

The Breakdown

Formula used in the calculator

Middle of the night = bedtime plus half the time from bedtime to wake-up. Gap = middle of the night on days off minus middle of the night on workdays. Sleep researchers call this gap social jetlag.

Worked example. A parent's usual week:

WorkdaysDays off
Bedtime11:00 pm12:30 am
Wake-up6:30 am9:00 am
Time in bed7.5 hours8.5 hours
Middle of the night2:45 am4:45 am

Days off put this person's sleep 2 hours later in the 24-hour cycle.

What the result means. Research on social jetlag describes how later bedtimes on free days also mean later light exposure, which can push the body clock later and leave it out of step when the workweek starts again. CDC and AASM both advise one wake-up time for every day, days off included, which keeps the gap small.

Circadian rhythm sleep-wake disorders. AASM groups several sleep disorders by timing. Four of them:

DisorderWhat happens
Delayed sleep-wake phaseSleep and wake-up times run much later than the usual hours
Advanced sleep-wake phaseAn "early birdChronotypeA person's natural lean toward earlier or later times for sleeping and being active, usually described as morning, evening or in between. See more" clock, with sleep coming several hours before a typical bedtime
Shift work disorderA work schedule clashes with the body clock, bringing insomnia, heavy sleepiness or both
Jet lag disorderTrouble adjusting to a new schedule after crossing several time zones

Where It Comes From

Chronobiologist Franz Halberg coined the word "circadian" in 1959 from the Latin circa, "about", and dies, "day". In 2017 Jeffrey C. Hall, Michael Rosbash and Michael W. Young shared the Nobel Prize in Physiology or Medicine for discovering molecular mechanisms that control the circadian rhythm, work built on a gene called period in fruit flies.

Good to Know

Circadian rhythm sleep-wake disorders are medical conditions a doctor diagnoses, and AASM's Sleep Education site covers them. If your sleep keeps landing at hours that clash with work or school, or shift work leaves you unable to sleep or stay alert, talk to a doctor or a sleep specialist. For everyday timing, AASM advises limiting bright light in the evening, which fits what NIGMS describes: the SCN times melatonin by how much light the eyes receive.

FAQ

What is a circadian rhythm disorder?

It's a sleep disorder in which the timing of sleep is out of step with day and night or with the hours a person has to keep. AASM's guideline covers delayed and advanced sleep-wake phase, irregular sleep-wake rhythm, non-24-hour sleep-wake rhythm, shift work disorder and jet lag disorder.

What controls your circadian rhythm?

A master clock in the brain called the suprachiasmatic nucleus, according to the National Institute of General Medical Sciences. It takes its timing mainly from light reaching the eyes, controls how much melatonin the brain makes, and keeps the clocks in other organs in sync.

How do you keep a steady circadian rhythm?

CDC and AASM both advise keeping bedtime and wake-up time the same every day, days off included. AASM also advises limiting bright light in the evening.

Do It in Buhata

Habit streaks

Add "Up at the same time" to Habit streaks (Plus) and tick it each morning, days off included.

Open Buhata

Related Terms and Categories

  • Sleep schedule

    The set times a person goes to bed and gets up each day, planned so they get the hours of sleep recommended for their age.

  • Seasonal affective disorder

    A type of depression that follows the seasons, most often starting in late fall or early winter and lifting in spring and summer.

  • Sleep debt

    The running total of sleep you've missed, counted night by night as the hours you need minus the hours you actually slept.

  • Sleep hygiene

    The everyday habits and bedroom setup that make it easier to fall asleep and stay asleep, like a steady wake-up time and a quiet, cool room.

Wellbeing and habits

Sources

National Institute of General Medical Sciences (NIH), "Circadian Rhythms" fact sheet; Etymonline, "circadian"; NobelPrize.org, "The Nobel Prize in Physiology or Medicine 2017"; American Academy of Sleep Medicine, clinical practice guideline for circadian rhythm sleep-wake disorders (2015 update); American Academy of Sleep Medicine, Sleep Education, "Advanced Sleep-Wake Phase"; American Academy of Sleep Medicine, "Circadian Adaptation to Shift Work"; "The association between social jetlag and poor health and its (nutritional) mechanisms" (PMC); CDC, "About Sleep"

Last updated .