May 30, 2026

What Is Circadian Rhythm? Your Body Clock Explained

Quick answer: A circadian rhythm is your body’s roughly 24-hour internal clock. It’s run by a master timekeeper in the brain—the suprachiasmatic nucleus—that takes its main cue from light and coordinates sleep, alertness, hormones, body temperature, and metabolism with the day–night cycle.

You feel your circadian rhythm every day: the afternoon dip, the second wind at night, the 3 a.m. wake-up, the morning grogginess after a late night. Understanding the body clock is the foundation for fixing almost any sleep or energy problem—and it’s the science Circady is built on.

Key takeaways

Circadian rhythm = your ~24-hour internal clock; ‘body clock’ is the everyday name for it.
The suprachiasmatic nucleus (SCN) in the brain is the master clock; light is its strongest cue.
It governs sleep, hormones (melatonin, cortisol), temperature, and metabolism.
Disruption affects sleep, mood, and—over time—metabolic health.
You keep it healthy with consistent light, sleep, and meal timing.

The master clock and its cues

Deep in the hypothalamus, the suprachiasmatic nucleus (SCN) keeps a rhythm close to—but not exactly—24 hours. Every day it re-aligns using external time cues called zeitgebers. Light is by far the most powerful: a systematic review concluded that the timing, intensity, and duration of light are the primary factors that entrain and shift the human clock (Tähkämö and colleagues, Chronobiology International, 2019). Special light-sensing cells in your eyes report ‘daytime’ to the SCN, which then sets the tempo for clocks in nearly every organ.

What your body clock controls

Sleep and alertness: melatonin rises in the evening; the cortisol awakening response peaks after you wake.
Metabolism: insulin sensitivity is higher earlier in the day.
Body temperature: dips at night to enable deep sleep, rises through the day.
Hormones and digestion: released on a daily schedule that anticipates your routine.

What happens when it’s disrupted

Late nights, evening screens, shift work, and travel pull your behavior out of sync with the clock. Beyond poor sleep and fog, this misalignment has measurable effects: in a controlled study, misaligning behavior from the body clock raised glucose and blood pressure and lowered the satiety hormone leptin (Scheer and colleagues, PNAS, 2009). This is the link between circadian health and metabolic health—and why timing matters as much as habits.

How to keep a healthy rhythm

Get bright light within an hour of waking; dim it at night.
Keep consistent wake and sleep times, including weekends.
Eat earlier in the day and stop eating 2–3 hours before bed.
Move your body in daylight; keep the bedroom cool and dark.

Circady applies this directly: matching nutrients to the time of day your body can use them. Explore Circady’s body-clock routine. Supplements support these daily habits—they don’t replace them.

Frequently asked questions

What is a circadian rhythm?

A circadian rhythm is your body's roughly 24-hour internal clock that controls sleep, alertness, hormone release, body temperature, and metabolism, syncing your biology to the day-night cycle.

What controls your circadian rhythm?

A master clock in the brain called the suprachiasmatic nucleus (SCN) sets the pace, taking its strongest cue from light. Meal timing, activity, and temperature provide supporting cues.

What is the body clock?

Body clock is the everyday term for your circadian rhythm: the internal timing system that tells you when to feel sleepy, hungry, and alert across 24 hours.

What happens when your circadian rhythm is disrupted?

Disruption from late nights, screens, shift work, or travel can cause poor sleep, daytime fatigue, and mood changes, and over time can affect metabolism and blood sugar.

How do you keep a healthy circadian rhythm?

Get bright light in the morning, keep consistent wake and sleep times, eat earlier in the day, stay active, and limit bright light and food late at night.

References

Tähkämö L, Partonen T, Pesonen AK. Systematic review of light exposure impact on human circadian rhythm. Chronobiology International, 2019. tandfonline.com
Scheer FAJL, Hilton MF, Mantzoros CS, Shea SA. Adverse metabolic and cardiovascular consequences of circadian misalignment. PNAS, 2009. pubmed.ncbi.nlm.nih.gov/19255424

Medically reviewed by Dr. Omar Saeed, PhD — May 29, 2026.

This article is for educational purposes only and is not a substitute for professional medical advice. These statements have not been evaluated by the Food and Drug Administration. Circady products are not intended to diagnose, treat, cure, or prevent any disease.