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# Circadian Rhythm

cir·ca·di·an rhy·thm - sur-KAY-dee-un RITH-um

## Definition

A circadian rhythm is a roughly 24-hour internal biological cycle that regulates many of your body's processes, including the sleep-wake cycle, [hormone](/glossary/hormone) release, body temperature, [metabolism](/glossary/metabolism), and even immune function. The word 'circadian' comes from the Latin circa (about) and diem (day). These rhythms are driven by a master clock in the brain - a cluster of about 20,000 neurons called the suprachiasmatic nucleus (SCN), located in the hypothalamus just above where the optic nerves cross.

Your circadian clock is primarily synchronized by light exposure. When light enters your eyes, specialized photoreceptors in the retina send signals to the SCN, which coordinates the timing of biological processes throughout the body. In the morning, light signals suppress melatonin production and increase [cortisol](/glossary/cortisol), promoting alertness. In the evening, diminishing light triggers [melatonin](/glossary/melatonin) release, preparing the body for sleep. This is why exposure to bright artificial light and screens at night can disrupt sleep - it confuses the circadian clock.

Circadian disruption [has](/glossary/hyaluronic-acid) become a significant public health concern in modern society. Shift work, jet lag, irregular sleep schedules, and excessive nighttime light exposure can desynchronize the body's internal clocks, leading to sleep disorders, metabolic dysfunction, mood disturbances, weakened immunity, and increased risk of chronic diseases including obesity, diabetes, [cardiovascular](/glossary/cardiovascular) disease, and certain cancers.

## Also Known As

Body clock Internal clock Sleep-wake cycle 

## Key Facts

-   • The master circadian clock (SCN) in the brain coordinates peripheral clocks in virtually every organ and tissue. 
-   • Circadian rhythms persist even in the absence of external time cues, running on a roughly 24.2-hour cycle. 
-   • Morning sunlight exposure is the most powerful tool for synchronizing your circadian rhythm. 
-   • Shift workers have a 17% increased risk of cardiovascular events compared to day workers. 
-   • Nobel Prize in Physiology or Medicine was awarded in 2017 for discoveries of the molecular mechanisms controlling circadian rhythm. 

## How It Relates To Your Health

Circadian rhythm disruption is now recognized as a contributor to numerous health conditions including insomnia, depression, bipolar disorder, metabolic syndrome, and cancer. If you struggle with sleep, your healthcare provider may evaluate your circadian patterns through sleep diaries, actigraphy (wrist-worn movement monitors), or measurement of dim-light [melatonin](/glossary/melatonin) onset.

Strategies for maintaining healthy circadian rhythms include consistent sleep and wake times (even on weekends), morning bright light exposure, limiting screen time in the 1-2 hours before bed, avoiding caffeine after early afternoon, and keeping your bedroom cool and dark. These practices, collectively known as sleep hygiene, are the first-line treatment for circadian-related sleep problems.

## Related Terms

[Melatonin](/glossary/melatonin)[Cortisol](/glossary/cortisol)[Sleep Cycle](/glossary/sleep-cycle)[REM Sleep](/glossary/rem-sleep)[Homeostasis](/glossary/homeostasis)

## Related Articles

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### Sleep Hygiene: 12 Evidence-Based Tips

2025-09-15

[![How to Build a Morning Routine That Actually Sticks](https://images.unsplash.com/photo-1506126613408-eca07ce68773?w=1200&h=630&fit=crop&fm=webp)](/wellness/morning-routine-that-sticks)

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### How to Build a Morning Routine That Actually Sticks

2026-01-08

## Sources

1.  [Circadian Rhythms](https://www.nigms.nih.gov/education/fact-sheets/Pages/circadian-rhythms.aspx) - National Institute of General Medical Sciences (NIH)
2.  [Circadian Rhythm Sleep Disorders](https://www.sleepfoundation.org/circadian-rhythm) - Sleep Foundation
3.  [Circadian rhythm disruption and health](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6691703/) - National Library of Medicine

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