Circadian Rhythms and Their Impact on Sleep

The Sleep-Wake Cycle and Its Role in Health

High angle view of couple sleeping on bed at home

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All species have a timing mechanism, popularly referred to as an "internal clock," that regulates periods of activity and inactivity. The mechanism, known as circadian rhythms, helps direct sleep patterns and the regular fluctuation of biological processes during a roughly 24-hour timetable. You will experience the circadian rhythm during times of the day when you are alert and productive and others when you are sleepy and non-productive.

The dysregulation of the circadian rhythm can trigger sleep problems, such as insomnia and daytime sleepiness, and can correlate with worsening symptoms of depression and bipolar disorder.

Although people tend to think of circadian rhythms as a single process, there are actually a collection of interrelated clocks that oscillate independently throughout the day. For example, mental alertness tends to peak twice daily at 9:00 a.m. and 9:00 p.m., while physical strength tends to crest at 11:00 a.m. and 7:00 p.m.


Earth’s 23-hour, 56-minute daily rotation provides predictable rhythms of light and temperature. Through adaptive evolution, our body’s metabolism and behaviors are programmed to respond to these effects.

In 1959, Dr. Franz Halberg coined the term circadian, taken from the Latin for "about a day." Halberg used the term to describe the phenomenon whereby activity and inactivity occurred in routine peaks and valleys, not only in humans but nearly every living organism on the planet.

Science has since revealed many of the underpinnings of this phenomenon, most especially the role of the sun and sunlight.

The sun and light are very important in the circadian rhythm as photosensitive cells in the retina are directly connected to the anterior hypothalamus gland where the suprachiasmatic nucleus (SCN), or the body's pacemaker, is located.

While the exact mechanisms of the SCN are unclear, environmental cues—most especially the sun—are central to the regulation of the daily sleep-wake stages.

In the course of evolution, humans and animal physiology adapted so that sunlight could penetrate the eyelids or the nictitating membrane covering the eyeballs. Sunlight entering the eye travels to the optic nerves, above which the X-shaped optic chiasm delivers the nerve signals to the SCN. Thereafter:

  • As sunlight increases the start of the day, the visual system signals the SCN to activate N-methyl-D-aspartate (NDMA) receptors in the brain. These receptors stimulate the production of the stress hormone cortisol, leading to wakefulness and increased energy.
  • As sunlight decreases at the end of the day, the visual system signals the SCN to activate the pineal gland, the organ responsible for producing the sleep hormone melatonin.

Environmental factors—most especially the morning sunlight—impact how the internal clock is set and reset. These external influences are referred to as zeitgebers (derived from the German for "time givers"). But, sunlight is not the only factor that influences the circadian rhythm; genetics also play a part.


Within the human body, the SCN synchronizes many of the biological and physiological processes needed to function normally. These include:

  • Sleep and wakefulness
  • Metabolism
  • Core body temperature
  • Cortisol levels
  • Melatonin levels
  • Other hormones levels (including growth hormone, thyroid hormone, etc.)

The regulation of these rhythms is built into our genetic makeup. These genes help maintain the circadian rhythm independent of outside forces, including the sun. Even if the sun doesn't rise one day, the circadian rhythm will still continue.

The first gene associated with the circadian rhythm, called CLOCK (Circadian Locomotor Output Cycles Kaput), was identified by Dr. Joseph Takahashi and colleagues in 1994.4 Multiple genes have since been identified that constitute the body's core molecular clock.

Every cell in our body follows a specific circadian pattern. Through hormones and other as-of-yet-undetermined influences, the SCN coordinates the peripheral clocks in every cell type of the body, including the heart, liver, lungs muscles, and kidneys.

These patterns will persist without external cues but can sometimes change based on changes in the geographical length of the day. One such example is jet lag in which the circadian rhythm is desynchronized as you move over multiple time zones. Other factors, such as daylight savings time, can lead to short-term desynchronization.

The degree of desynchronization is largely dependent on an individual's genetics (referred to as tau) and the extent to which day and nighttime patterns are interrupted.

Circadian Desynchronization

When the internal clock is misaligned to one's environment or life schedule, circadian disorders such as delayed sleep-wake phase syndrome (in which you are unable to fall asleep) and advanced sleep-wake phase syndrome (in which sleep occurs prematurely) can develop.

When completely deprived of light, as occurs with total blindness, a condition known as non-24 sleep-wake rhythm disorder (non-24 SWRD) can also develop.

Research has shown that people who are blind from birth frequently have difficulty with their sleep-wake cycle because of the lack of environmental light cues. This, in turn, alters the pattern of alertness, mood, and performance compared to others around them. Some blind people with a non-24 SWRD may even feel that the condition is far more disabling than blindness itself.

Non-24 sleep-wake phase disorder can also occur in sighted individuals, albeit rarely. The cause is not entirely clear, but it seems to affect those with extremely irregular work hours, including those with ongoing shift work.

People who live close to the northern or southern poles are also prone to circadian disorders due to profound seasonal changes in day and night patterns. A fairly large study compared 453 Norwegians (319 males and 134 females) and 450 Russians (317 males and 133 females) living on Svalbard. The Russians, but not the Norwegians, reported more problems during the polar night. Of the Russians, 81% of the men and 77% of the women reported sleep problems (difficulty falling asleep, night awakenings, not feeling rested). The authors considered that this may have been due to the greater recruitment of Russians from lower latitudes compared to the Norwegians.

Morning People vs. Night People

Irrespective of environmental influences, there are people who clearly thrive in the morning hours and others who seem at their best after dark. Many of these behaviors appeared to be learned rather than directed by the environment or genetics.

Although "night people" appear to be more productive and alert at night, a growing body of research suggests that they may not be living the healthiest of lifestyles.

A 2018 study published in Advances in Nutrition reported that night people may be more likely to have a heart attack and type 2 diabetes compared to early risers, likely due to poor dietary choices.

They were also more likely to have sleep problems or to spend the weekends catching up on sleep.

By contrast, early risers tended to be more physically active, eat better diets, and have a reduced need for calories in the evening when metabolism tends to slow. As a result, they were less likely to develop metabolic syndrome than their night-loving counterparts.

Interestingly, that study found that early risers and night owls had one distinct thing in common: they both were most productive in the early morning hours.

Overcoming Circadian Rhythm Disorders

Irregular sleep patterns not only interfere with your health but can compromise the quality of your home, work, and social life. This is especially true of night owls or people who are forced to work overnight shifts.

To reduce work-related stress, night owls are often advised to spend a few early-morning hours doing vital tasks whenever possible. This may help reduce stress-related eating that contributes to weight gain and poor glucose control. It also allows for more time to exercise at night.

If faced with insomnia or non-24 SWRD, a nightly 5 to 10 mg dose of melatonin has been known to improve sleep patterns. Studies have even demonstrated the effectiveness of doses as low as 0.5mg. If melatonin does not work, the prescription drug Hetlioz (tasimelteon) can also be useful in blind individuals with non-24 SWRD.

Ultimately, the best way to overcome circadian dysregulation is to reset your internal clock. Most people can do this by improving their sleep hygiene. Here are a few simple tips that can help:

  • Maintain a regular sleep schedule. Go to bed at the same time every night of the week and use an alarm clock to wake yourself up at the same time every morning.
  • Avoid daytime napping. Napping decreases your "sleep debt," meaning that you need less sleep at night. This can interfere with a regular sleep routine.
  • Don’t watch TV or read in bed. Stop any form of entertainment and turn off all electronics (including cell phones) at least 30 minutes before your scheduled bedtime.
  • Avoid caffeine and alcohol several hours before sleep. Caffeine can overstimulate you. Alcohol may help you fall asleep but is more likely to cause sleep interruptions and morning grogginess.
  • Keep the bedroom dark. Turn off all lights and close the bedroom curtains so that the room is very dark. Although some people will recommend sleep masks, they can prevent sunlight from entering the eye and impede the environmental signals meant to stir you.
  • Keep the temperature cooler. Cooler temperatures tend to aid with sleep. Even in the winter, avoid overheating yourself with too many blankets.
  • Keep it quiet. Even though you may think you can sleep through anything, sudden noises can cause momentary starts that interrupt otherwise restful sleep. If your partner snores loudly, explore anti-snore remedies or invest in a pair of earplugs.

Most sleep experts will tell you that it can take a month or more to establish a new wake/sleep routine. But, with a little perseverance and dedication, you may soon reap the benefits of being a full-fledged morning person.

A Word From Verywell

The causes of circadian disorders are not always clear and may take more than melatonin to set things right. If faced with chronic insomnia and daytime sleepiness, ask your doctor for a referral to a board-certified sleep physician who can help diagnose and treat your condition.

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  5. Lockley SW, Dressman MA, Licamele L, et al. Tasimelteon for non-24-hour sleep-wake disorder in totally blind people (SET and RESET): two multicentre, randomised, double-masked, placebo-controlled phase 3 trialsLancet. 2015;386(10005):1754-64. doi:10.1016/S0140-6736(15)60031-9

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