Circadian Rhythm


Written by Matthew Whittle

Reviewed by Dr. Michael Breus

Our Editorial Process

Table of Contents

Circadian rhythms are physical and mental processes that fluctuate across each daily 24-hour cycle. The 24-hour circadian rhythm is important because it helps us be alert during the day and ready for sleep at night. It also influences our digestive systems, hormones, and even body temperatures. 

Sometimes, our circadian rhythms become misaligned with our sleep schedule and environment. Knowing more about your sleep-wake cycle and common disruptions to your circadian rhythm can ensure you are getting the quality sleep you need. We also cover tips for building and maintaining a healthy sleep-wake cycle.

Key Takeaways


    • Circadian rhythms regulate our internal clock, notably our sleep-wake patterns.
    • Exposure to bright light in the morning and darkness in the evening helps synchronize your sleep-wake schedule.
    • Natural light, a regular sleep schedule, and exercise are key to maintaining a healthy circadian rhythm.

What Is Your Circadian Rhythm?

Circadian rhythms are biological patterns that shift across each 24-hour period. One of the most noticeable circadian rhythms is the sleep-wake cycle, which determines a person’s level of sleepiness or wakefulness throughout the day and night.

Not everyone’s circadian rhythm is the same. For people who are “night owls,” a normal daily rhythm involves staying up at night and sleeping late in the morning. Other people fall asleep soon after dark and are up with the sun. These people are known as “early birds” or “larks.”

Variations in circadian rhythms are referred to as chronotypes. Research suggests that chronotypes are likely based on factors like age, genetics, daily light exposure. If you’re interested in learning your chronotype, you can take a quiz and find out how chronotype affects your sleep-wake cycle. 

How Does the Circadian Rhythm Affect Sleep?

A person’s sleep patterns are heavily influenced by their sleep-wake cycle. Complex interactions in the brain determine how awake or sleepy you feel at a given time. 

The body’s internal clock is located in the brain. This part of the brain is called the suprachiasmatic nucleus (SCN). The SCN is located close to the eyes and is sensitive to light from the environment. The internal clock synchronizes circadian rhythms throughout the body, including the sleep-wake cycle, in response to the light of daytime and the darkness of nighttime. 

During the day, the SCN reacts to light by keeping a person alert. At night, the reverse is true. When the environmental light dims, the SCN signals the brain to produce melatonin, a hormone that makes you feel sleepy. 

It’s best if people get sunlight in the morning, within 30 minutes of waking up. Vitamin D is a circadian pacemaker, so being outside in the sun, even for a few minutes, actually helps you get better sleep.
Dr. Michael Breus

If your sleep-wake cycle is not in alignment with natural light and darkness cues, you may notice a range of changes to your sleep health, and consequently, your physical health. Insufficient sleep can cause mood changes, trouble focusing, and accidents. 

While light is the primary cue that affects the sleep-wake cycle, other factors like how long you’ve been awake can also play a role in guiding your sleep patterns. 

What Can Disrupt a Circadian Rhythm?

Circadian rhythms can be disrupted by changes in our environment, our daily schedules, and our bodies.

  • Daylight saving time: About a quarter of the people in the world experience time shifts related to daylight saving time. Although these shifts only move the clock by one hour, research shows that there is a short-term increase in fatal car accidents, workplace injuries, and heart attacks, which may be due to circadian disruptions.  
  • Jet lag: Jet lag occurs when a traveler’s circadian rhythm becomes out of sync with the  destination time zone. Jet lag tends to be more severe for those who have crossed several time zones. It can make travelers feel sleepy during the day and awake at night.
  • Shift work: Shift workers are people who work night shifts or very early morning shifts. They often experience circadian rhythm disruptions because they are required to be alert when it is dark outside and to sleep during daylight hours.

Circadian Rhythm Disorders

A number of sleep disorders are caused by differences in an individual’s circadian rhythms. While sleep problems associated with jet lag and shift work are based on changes in the environment, these circadian sleep-wake rhythm disorders have an internal basis. 

  • Non-24-hour sleep-wake syndrome: People with non-24-hour sleep-wake syndrome are unable to regularly align their sleep schedule with their external environment. This is most common in those who are blind, but it can affect sighted people as well.
  • Delayed sleep-wake phase disorder: Common in adolescents, people with delayed-sleep-wake phase syndrome prefer being a night owls. Although it is normal for many people to stay up late on occasion without consequences, delayed-sleep-wake phase disorder often interferes with work, school, or family responsibilities. 
  • Advanced sleep-wake phase disorder: People with advanced sleep-wake disorder fall asleep unusually early, then wake up very early in the morning. This sleep disorder mostly occurs in older people.
  • Irregular sleep-wake rhythm disorder: People with irregular sleep-wake rhythm disorder do not have any specific sleep-wake cycle. Because of this, they may have insomnia at night, short sleep periods, and an urge to nap during the day. This disorder most often occurs in people with dementia or developmental differences. 

Tips to Maintain the Circadian Rhythm That Regulates Sleep

If you feel like you need to fix your circadian rhythm, start by making sure you’re practicing healthy sleep hygiene habits. There are several strategies that can help you improve your sleep and maintain your sleep-wake cycle. 

Control Your Light Exposure

Be intentional about your light exposure so that you get plenty of bright light at the beginning of your day, and limit your exposure to light as you approach bedtime. 

Light is the most powerful factor in aligning your circadian rhythm. Therefore, exposure to bright light during the daytime can help improve your sleep cycle. Your circadian clock is especially sensitive to sunlight during the first hour after awakening, so open your curtains or blinds or make time to step outside in the morning. 

Exposing yourself to darkness in the evening and nighttime can also align your circadian clock. Avoid bright light in the evening, and keep your sleeping environment as dark as possible. Some research suggests that having even a dim light in the bedroom at night can affect circadian rhythms and cause subtle changes in the structure of sleep. 

Cell phones, TVs, and other electronic devices that emit blue light are particularly disruptive to sleep and should be avoided before bedtime. Cells in the eyes that respond to light are particularly sensitive to blue light, so using these devices in the hours before bed might signal to your brain that you should be alert rather than sleepy.

Stick With a Regular Sleep Schedule

Maintaining a consistent sleep and wake schedule can help keep your circadian rhythm on track. It is best to go to bed and wake up at the same time each day, even on weekends. Research shows that having a stable daily routine helps people get to sleep faster, spend less time awake in bed, and generally experience better sleep quality. 


Exercise can help you sleep better and align your circadian rhythm when it is out of sync. It is best to exercise earlier in the day and to avoid vigorous activity in the last few hours before your bedtime.

Frequently Asked Questions About Circadian Rhythm

Is circadian rhythm linked to obesity?

Working or socializing during standard sleep times may lead to weight gain for some people. This is because you may end up eating at times when your body is unable to metabolize food effectively. If your body isn’t metabolizing food well, you may burn up less energy overall, which can lead to weight gain.

Does depression affect your circadian rhythm?

There is an association between depression and a person’s circadian rhythm, but the relationship is complex. Essentially, sleep disruptions or changes in the sleep-wake cycle can lead to depression. But, depression can also cause disruptions to a person’s sleep-wake cycle.

Fortunately, people who take medication to treat their depression or use bright light therapy may see improvements in both their mood and sleep health.

If you or a loved one lives with depression and have concerns about sleep health, it is important to talk with a health care provider who can help you explore treatment options.

Do pets have circadian rhythms?

Like all animals, household pets like cats and dogs have circadian rhythms. These circadian rhythms affect a number of physical processes, such as when cats’ eyes produce the most tears. However, circadian rhythms appear differently in pets because their sleep-wake cycles vary from ours.

For example, cats have an internal clock regulating their circadian rhythms just as humans do. It controls their sleep cycles, body temperature, and activity levels. However, unlike humans, cats have multiple short sleep cycles, rather than one full night of sleep.

Like humans, dogs do most of their sleeping at night. However, they tend to sleep in shorter bouts, and frequently nap during the daytime.

About The Author

Matthew Whittle

Staff Writer

Matt Whittle is a freelance writer and editor who holds a B.A. in English from Penn State University. He has experience as a writer and editor in the wine industry and higher education.

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