Circadian rhythms are physical, mental, and behavioral changes that follow a 24-hour cycle. Typical circadian rhythms lead to high energy during the middle of the day (10 a.m. to 1 p.m.) and an afternoon slump. At night, circadian rhythms cause the hormone melatonin to rise, making a person sleepy.

Learn more in NIGMS’ circadian rhythms featured topics page.

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