The Complete Guide to Circadian Rhythm

Long Story Short

• Circadian rhythm, also known as the sleep-wake cycle, is the internal clock that regulates the body’s sleep and wake cycles over a 24-hour time period.
• The circadian rhythm also regulates many physiological functions, including blood pressure, core temperature, and metabolism.
• Changes to your circadian rhythm that shift your sleep and meal times can impact hormonal balance, including leptin and ghrelin, which regulate hunger (1)

If you’ve ever felt energized or tired around the same time each day, you’ve experienced your circadian rhythm at work. To catch you up here, your circadian rhythm is an internal clock that regulates your body’s sleep and wake cycles over a 24-hour period. And because your circadian rhythm is controlled by the part of your brain that responds to light, you’ll probably notice that your alertness tends to peak during the day and feel more blunted at night. (2)

While we know that our circadian rhythm plays a critical role in our sleep and wake cycles, its importance doesn’t end there. Your circadian rhythm interacts with your brain and body to regulate numerous essential physiological functions. From body temperature homeostasis to metabolism, your circadian rhythm is hard at work to keep your body in good working order. (3) 

Note: The content on Sleepopolis is meant to be informative in nature, but it shouldn’t be taken as medical advice, and it shouldn’t take the place of medical advice and supervision from a trained professional. If you feel you may be suffering from any sleep disorder or medical condition, please see your healthcare provider immediately.

What is the Circadian Rhythm?

Mark Aloia, head of sleep and behavioral sciences at Sleep Number, tells Sleepopolis, “Circadian rhythm is our internal clock that runs on a 24-hour cycle. It regulates our bodies’ sleep-wake cycles and dictates the timing and regularity of much of what we do.” This includes the host of physical, mental, and behavioral changes we experience over a 24-hour period. (4) 

“The circadian rhythm determines when you sleep, but it also influences when you’re the most alert, most likely to perform well, when you should eat, and when you’re in the best mood,” says Aloia. Circadian rhythm also plays a role in: (3)

• Blood pressure/Cardiovascular health 
• Core body temperature
• The immune system
• Hormone secretion
• Metabolism
• Appetite 
• Cognitive function
• Digestion
• Energy balance
• The body’s reaction to stress

Circadian rhythm isn’t limited to humans. Plants, animals, and even microorganisms have circadian rhythms, too. (4)

How did we get here?

Research into our circadian rhythm first began in the 18th century, when scientist Jean-Jacques d’Ortous de Mairan observed a plant opening and closing its leaflets each day in complete darkness, suggesting the existence of a biological clock. (5) 

Two hundred years later, not only have scientists proven that circadian rhythms do, in fact, synchronize with light-dark cycles, but they also discovered that genes inform circadian rhythms. (6)

How Does the Circadian Rhythm Work?

The circadian rhythm (aka your body’s master clock) is controlled by the suprachiasmatic nucleus (SCN) — a cluster of approximately 20,000 nerve cells in the hypothalamus. (7)

Aloia says, “There are a few factors that play into our circadian system, and light is one of the strongest ones.” 

Each day, our eyes track changing light levels in our environment, which signals the SCN to synchronize our internal clocks with the external environment and trigger certain bodily functions in response. (8) 

When it comes to the circadian rhythm and sleep, the most simple way to explain it is this: light tells your brain when it’s time to be awake, and darkness lets your brain know it’s time to shift to sleep mode — more on that below. (9)

Circadian Rhythm and Sleep 

As we mentioned earlier, circadian rhythms play a key role in our sleep and wake times — if you’re wondering how that all works exactly, here’s the science. Light serves as the cue that syncs things up. Aloia explains that with light exposure (a la sunrise), our melatonin production is suppressed, ultimately signaling the body to be awake and alert. Conversely, a lack of light (a la sunset) flips the switch, releasing melatonin to help our bodies feel tired and prepare for sleep. (9) Your core body temperature (regulated by your circadian rhythm) also drops around this time to decrease your alertness in preparation for bed. (10) 

If all goes as planned, we will have a smoothly operating circadian rhythm and consistent sleep schedule. However, Aloia notes, “If light exposure is used at times that aren’t aligned with your circadian rhythm, it can throw off your sleep schedule and other biological processes.” 

We’ll add here that while light is the strongest cue, other signals that influence your circadian rhythm are: (11)

• Food/Meal timing
• Exercise
• Stress
• Social interactions 

Circadian rhythm is responsible for keeping our sleep cycles in check and our biological processes ticking and working in harmony, so misalignments between our circadian rhythm and our environment can wreak a little havoc on our sleep. We’ll discuss this in more detail later, but a circadian rhythm that’s out of whack can lead to delayed sleep onset (makes it harder to fall asleep), frequent nighttime wakings, and short sleep overall. (12)

Common Disruptions of the Circadian Rhythm 

Natalie D. Dautovich, Ph.D., an environmental fellow with the National Sleep Foundation, tells us that circadian rhythm disturbances basically look like sleep disturbances. She says, “Common disruptions of circadian rhythms include a phase advance, where someone wakes up extremely early and falls asleep early, or a phase delay, where a person cannot fall asleep until late and is sleepy later in the morning.” (13) (14)

Essentially, Dautovich explains that if a person finds that they have difficulty being awake when they need to be awake or asleep when they would like to sleep, that could indicate a phase disruption of the circadian system or, at the very least, another sleep or medical disorder.

As to what may spark circadian rhythm disruptions, its delicate balance can be thrown off-kilter by a host of variables, including: (15) (16) (17) 

• Traveling across time zones
• Shift work
• Late socializing or study habits 
• Illnesses
• Rousing sleep environment (noisy, bright)
• Stress
• Any type of frequent sleep disruptions 

The net effect of a circadian disruption will typically look remarkably similar to sleep deprivation, the symptoms of which look different over the short and long term. 

Over the short term, circadian rhythm disruptions may translate to: (15) (18) (19)

• Sleep problems 
• Moodiness/Irritability 
• Fatigue 
• Impaired work or academic performance 

Over the long term, circadian rhythm disruptions may translate to more serious and chronic health problems, both physical and emotional, like: (15) (20)

• Anxiety 
• Depression 
• Obesity 
• Hypertension 
• Stroke 

How To Fix Your Circadian Rhythm 

Whether it’s a result of traveling for vacations, picking up a few night shifts, or moving to a new apartment in the city, many of us will likely experience disruptions to our circadian rhythm at some point in our lives. Miscalibrations are to be expected, but there are some things you can do to restore the balance. This includes: (13)

• Maintaining a consistent sleep schedule
• Photic treatment – which essentially boils down to morning bright light exposure and avoiding bright and blue light in the evening before bed
• Melatonin supplements
• Exercise 
• Improved sleep hygiene 

Circadian Rhythm Disorders

Circadian rhythm disorders (also known as circadian rhythm sleep-wake disorders, or CRSWDs) are a series of sleep disorders that occur when the body’s internal clock is out of sync with the external environment. (12) We’ll dig into some of the more common ones below. 

Delayed Sleep Phase Syndrome (DSPS)

Someone is said to have delayed sleep-wake phase disorder (DSWPD) when they only ever feel tired enough to go to sleep and wake up later than what’s considered normal. (12) Typically, those with DSWPD will go to bed two hours (or more) later than average. (12) DSPS is quite common among adolescents, as this segment of the population typically experiences a later shift in their circadian rhythms around puberty. (21)

Considering that most of society operates on a nine-to-five schedule, those with DSPS usually have a hard time “getting along” as the clock in the world around them doesn’t quite jibe with their sleep schedules. Ultimately, the net effect for these folks is persistent short sleep. If those with DSPS try to make it work, they’ll likely experience excessive daytime sleepiness, fatigue, irritability, and cognitive impairments. (12)

When allowed to lean into their chronotype, these things may not be an issue, as those with DSPS can fulfill their sleep requirements more regularly. (12)

Common treatments for DSPS include chronotherapy (the patient is tasked with maintaining a consistent sleep-wake schedule and photic treatment, which includes exposure to bright light in the morning and avoidance of light exposure in the evening. Other treatments may include melatonin supplementation or cognitive behavioral therapy. (12)

Advanced Sleep-Wake Phase Disorder (ASWPD)

On the other side of the spectrum, those with advanced sleep-wake phase disorder (ASWPD) typically go to bed and wake earlier than what’s considered normal. (12) They are likely to go to bed two hours earlier than average, and as a result of their early bedtime, they’ll also wake up two hours earlier than average. (12)

Late afternoon and early evening drowsiness and pre-dawn wake-ups with difficulty going back to sleep are not uncommon for those with ASWPD. (12) Those with ASWPD are usually able to fall in line with the world around them, but their early bedtime may impede a robust social calendar. (12)

Common treatments for ASWPD include chronotherapy, photic treatment, and instruction on improved sleep hygiene. (12)

Non-24-Hour Sleep-Wake Syndrome

Non-24-hour sleep-wake disorder (N24) is a sleep disorder in which the body’s biological clock fails to synchronize to a 24-hour day. (12) Instead of hitting the hay at roughly the same time each night, those with N24 have delayed sleep times (ranging anywhere from a few minutes to hours) with each passing day. As a result, folks with N24 will find that their sleep start times inch later and later until they make their way around the clock. (12)

One notable aspect of N24 is that those dealing with the disorder typically experience symptomatic and asymptomatic phases. (12) At some point, their sleep-wake cycle moves so far back that it comes full circle, realigning with the standard 24-hour light-dark cycle most of us work on. (12) However, this “breather” is typically short-lived, as their sleep cycle continues to shift later and later with each turn of the calendar page. (12) 

N24 is relatively rare in the sighted population and quite common in people who are blind, as the retina cannot transmit natural light to the hypothalamus portion of the brain. As much as 50 percent of blind people exhibit signs and symptoms of N24. (22)

Treatments for N24 will vary based on levels of light perception. Those with light perception may be treated with chronotherapy of melatonin supplementation, while those with no light perception are more likely to be treated with photic treatment or medication. 

Treatments N24 include chronotherapy and photic treatment. (12)

Irregular Sleep-Wake Rhythm Disorder

This rare circadian rhythm disorder impacts people with sleep-wake cycles that don’t fit within the usual 24-hour clock. (12) Those with irregular sleep-wake rhythm disorder (ISWRD) can nap for as many as 7-9 hours each day. In turn, this may cause excessive daytime sleepiness due to circadian misalignment, not to mention unusual sleep patterns. (12)

There are no current estimates on the prevalence of irregular sleep-wake rhythm disorder, and sleep experts and researchers do not yet understand its development. One working theory is that it may develop in people with a weak body clock, which may result from neurological conditions such as dementia and brain damage. (12)

Common treatments for ISWRD include chronotherapy, photic treatment, evening melatonin supplementation, behavioral therapy, or a combination of any of the above. (12)

Shift Work Sleep Disorder

Shift-work sleep disorder (SWSD) results when there’s a conflict between a person’s circadian rhythm and work schedule. (12) Essentially, their work schedule runs counter to their internal body clock, often leading to insomnia and excessive daytime sleepiness. (12)

“Shift work” is defined as working hours outside of the traditional 9 a.m. to 5 p.m. time frame. Ultimately, as a result of the misalignment between the sleep-wake and dark-light cycles, SWSD sufferers may have difficulty falling asleep, staying asleep, or sleeping when tired. Approximately 20 percent of workers in the U.S. are shift workers, and as many as 26.5 percent may suffer from SWSD. (23) 

Jet Lag

You might have noticed upon your return from Japan last year that you felt tired, foggy, and out of sorts during the following week — in all likelihood, that was jet lag. Jet lag is a glitch in your circadian rhythm that usually occurs when you travel across several time zones in a relatively short period of time. (24) The most common symptoms of jet lag are: (24)

• Disturbed sleep 
• Excessive daytime sleepiness/Fatigue 
• Fogginess, confusion, and difficulty focusing on work or school 
• Headaches 
• Irritability 
• Heightened anxiety
• Gastrointestinal distress

Jet lag can feel pretty awful (see above), but rest assured that its effects are temporary, usually resolving themselves relatively quickly post-travel. 

Tips For Maintaining A Healthy Circadian Rhythm

Circadian rhythms are responsible for keeping our biological processes ticking and working in harmony. They also help us anticipate changes in our environment (like changes in temperature and food scarcity or availability) and respond accordingly. (8) And while life happens, you can ensure your circadian rhythm keeps time by practicing good sleep hygiene. Ahead, we’ve outlined some other ways to keep your circadian rhythm healthy. 

Stick to a consistent sleep schedule. Your circadian rhythm relies on regularity for optimal performance, so be sure to maintain a consistent sleep schedule. Go to bed at the same time each night and wake up at the same time every morning, including weekends and holidays, if you can. (13)

Go for a morning walk. Morning sun exposure can boost alertness and do some pretty heavy lifting to regulate your circadian rhythm. (25) You might think about adding a morning stroll to your daily routine, or having your morning cup of coffee outside. 

Limit evening technology use. For many people, scrolling through social media is an excellent bookend to the day. However, studies show that blue light from phones or other devices has a powerful suppressive effect on melatonin production and can ultimately undermine sleep quality. (26) 

Time your meals right. A growing body of research suggests the body functions optimally when eating patterns align with circadian rhythms. Studies say eating earlier in the day and having a light meal around dinner time is optimal for metabolic health. (27)

Optimize your sleep environment. The ideal sleep environment is one that’s cool, dark, and quiet. (28) So, consider making changes as needed. Think about blackout curtains, sleep masks, sleep headphones, or white noise machines. 

Reduce stress. Stress responses vary throughout the day and can disrupt the circadian clock. (29) To bring yourself and your circadian rhythm back into balance, consider meditation, tai chi, journaling, breathing techniques, or therapy as ways to reduce stress.

Circadian Rhythm and Chronotypes

If the concept of circadian rhythm still seems a little abstract, the topic of chronotypes may focus the lens a bit. 

We’ve all heard the terms “early bird” and “night owl.” (Some of us even use these phrases to describe our own sleep habits.)  And while they’re often mocked as pseudo-science, the chronotypes are very real. (30) Long story short, your chronotype is an expression of your circadian rhythm or a (fun) way to describe how your internal clock behaves. 

There are four main chronotypes that are most commonly referred to, and they were introduced by Dr. Michael Breus, PhD, in 2016.

• Bears. Bears tend to sleep and wake according to the sun. They typically feel most energetic during the day and fall asleep easily at night.
• Wolves (Night Owls). Wolves generally sleep and wake later than others. They are often most productive in the middle of the day and in the evening.
• Lions (Early Birds). Lions wake up early and are most productive in the morning. Due to early wake times and daytime energy expenditure, they are usually tired by evening.
• Dolphins. Dolphins are light sleepers who have difficulty following a regular sleep routine. They are most productive in the mid-morning to early afternoon.

Not sure what chronotype you are? Take our quick chronotype quiz to find out!

FAQs

The Last Word from Sleepopolis 

Circadian rhythms are powerful biological cycles that impact nearly every aspect of health and wellness. They determine sleep patterns and influence hormone release, eating habits and digestion, body temperature, and other important biological functions. Maintaining consistent sleep and wake times, limiting exposure to blue light at night, and sticking to a consistent eating pattern is key to keeping your circadian rhythm in tip-top shape. 

Sources

1. Boege, H. L., Bhatti, M. Z., & St-Onge, P. (2021). Circadian rhythms and meal timing: Impact on energy balance and body weight. Current Opinion in Biotechnology, 70, 1. https://doi.org/10.1016/j.copbio.2020.08.009
2. von Gall C. The Effects of Light and the Circadian System on Rhythmic Brain Function. Int J Mol Sci. 2022;23(5):2778. Published 2022 Mar 3. doi:10.3390/ijms23052778
3. Yeliz Serin, Nilüfer Acar Tek; Effect of Circadian Rhythm on Metabolic Processes and the Regulation of Energy Balance. Ann Nutr Metab 28 May 2019; 74 (4): 322–330. https://doi.org/10.1159/000500071
4. U.S. Department of Health and Human Services. (n.d.-b). Circadian rhythms. National Institute of General Medical Sciences. https://www.nigms.nih.gov/education/fact-sheets/Pages/circadian-rhythms.aspx 
5. Huang RC. The discoveries of molecular mechanisms for the circadian rhythm: The 2017 Nobel Prize in Physiology or Medicine. Biomed J. 2018;41(1):5-8. doi:10.1016/j.bj.2018.02.003
6. Ashbrook, L.H., Krystal, A.D., Fu, YH. et al. Genetics of the human circadian clock and sleep homeostat. Neuropsychopharmacol. 45, 45–54 (2020). https://doi.org/10.1038/s41386-019-0476-7
7. Ma MA, Morrison EH. Neuroanatomy, Nucleus Suprachiasmatic. [Updated 2023 Jul 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK546664/
8. Reddy S, Reddy V, Sharma S. Physiology, Circadian Rhythm. [Updated 2023 May 1]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK519507/
9. Tan DX, Reiter RJ, Zimmerman S, Hardeland R. Melatonin: Both a Messenger of Darkness and a Participant in the Cellular Actions of Non-Visible Solar Radiation of Near Infrared Light. Biology (Basel). 2023;12(1):89. Published 2023 Jan 6. doi:10.3390/biology12010089
10. Harding, E. C., Franks, N. P., & Wisden, W. (2020). Sleep and thermoregulation. Current Opinion in Physiology, 15, 7-13. https://doi.org/10.1016/j.cophys.2019.11.008
11. Shen, B., Ma, C., Wu, G., Liu, H., Chen, L., & Yang, G. (2023). Effects of exercise on circadian rhythms in humans. Frontiers in Pharmacology, 14, 1282357. https://doi.org/10.3389/fphar.2023.1282357
12. Sun SY, Chen GH. Treatment of Circadian Rhythm Sleep-Wake Disorders. Curr Neuropharmacol. 2022;20(6):1022-1034. doi:10.2174/1570159X19666210907122933
13. professional, C. C. medical. (n.d.-c). Time difference: When your circadian rhythm doesn’t Sync up. Cleveland Clinic. https://my.clevelandclinic.org/health/diseases/12115-circadian-rhythm-disorders#symptoms-and-causes 
14. The Lancet, 08 April 2023, volume 401, issue 10383, pages 1131-1240, … (n.d.-a). https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(24)01296-0/abstract 
15. professional, C. C. medical. (n.d.-a). Sleep deprivation: What it is, symptoms, treatment & stages. Cleveland Clinic. https://my.clevelandclinic.org/health/diseases/23970-sleep-deprivation 
16. Vetter, C. (2020). Circadian disruption: What do we actually mean? European Journal of Neuroscience, 51(1), 531-550. https://doi.org/10.1111/ejn.14255
17. Steinach, M., & Gunga, H.-C. (1970, January 1). Circadian rhythm and stress. SpringerLink. https://link.springer.com/chapter/10.1007/978-3-030-16996-1_9 
18. Walker, W.H., Walton, J.C., DeVries, A.C. et al. Circadian rhythm disruption and mental health. Transl Psychiatry 10, 28 (2020). https://doi.org/10.1038/s41398-020-0694-0
19. Mehta, K.J. Effect of sleep and mood on academic performance—at interface of physiology, psychology, and education. Humanit Soc Sci Commun 9, 16 (2022). https://doi.org/10.1057/s41599-021-01031-1
20. Ahmad, M., Balqis, N. S., Tharumalay, R. D., Che Din, N., Ibrahim, N., Amit, N., Farah, N. M., Osman, R. A., Abdul Hamid, M. F., Ibrahim, I. A., Jamsari, E. A., Palil, M. R., & Ahmad, S. (2020). The Effects of Circadian Rhythm Disruption on Mental Health and Physiological Responses among Shift Workers and General Population. International Journal of Environmental Research and Public Health, 17(19), 7156. https://doi.org/10.3390/ijerph17197156
21. Gradisar, M., Kahn, M., Micic, G. et al. Sleep’s role in the development and resolution of adolescent depression. Nat Rev Psychol 1, 512–523 (2022). https://doi.org/10.1038/s44159-022-00074-8
22. Emens, J. (1970, January 1). Non-24-hour sleep-wake rhythm disorder. SpringerLink. https://link.springer.com/chapter/10.1007/978-3-030-43803-6_9 
23. Pallesen, S., Bjorvatn, B., Waage, S., Harris, A., & Sagoe, D. (2021). Prevalence of Shift Work Disorder: A Systematic Review and Meta-Analysis. Frontiers in Psychology, 12, 638252. https://doi.org/10.3389/fpsyg.2021.638252
24. Roach, G. D., & Sargent, C. (2019). Interventions to Minimize Jet Lag After Westward and Eastward Flight. Frontiers in Physiology, 10, 462502. https://doi.org/10.3389/fphys.2019.00927
25. Choi, K., Shin, C., Kim, T., Chung, H. J., & Suk, H. (2019). Awakening effects of blue-enriched morning light exposure on university students’ physiological and subjective responses. Scientific Reports, 9(1), 1-8. https://doi.org/10.1038/s41598-018-36791-5
26. Pham HT, Chuang HL, Kuo CP, Yeh TP, Liao WC. Electronic Device Use before Bedtime and Sleep Quality among University Students. Healthcare (Basel). 2021;9(9):1091. Published 2021 Aug 24. doi:10.3390/healthcare9091091
27. BaHammam AS, Pirzada A. Timing Matters: The Interplay between Early Mealtime, Circadian Rhythms, Gene Expression, Circadian Hormones, and Metabolism-A Narrative Review. Clocks Sleep. 2023;5(3):507-535. Published 2023 Sep 6. doi:10.3390/clockssleep5030034
28. Iao SI, Jansen E, Shedden K, et al. Associations between bedtime eating or drinking, sleep duration and wake after sleep onset: findings from the American time use survey. Br J Nutr. Published online September 13, 2021. doi:10.1017/S0007114521003597
29. Phan TX, Malkani RG. Sleep and circadian rhythm disruption and stress intersect in Alzheimer’s disease. Neurobiol Stress. 2018;10:100133. Published 2018 Oct 17. doi:10.1016/j.ynstr.2018.10.001
30. Kalmbach DA, Schneider LD, Cheung J, et al. Genetic Basis of Chronotype in Humans: Insights From Three Landmark GWAS. Sleep. 2017;40(2):zsw048. doi:10.1093/sleep/zsw048