Most people feel tired at night and hungry at lunchtime. That’s because our bodies have a built-in process that regulates sleep, hunger, and energy levels -- the circadian rhythm. Although the cycle can be affected by artificial lights or travel, unless a health issue is at play, our 24-hour "clock" always resets itself in time. We share this rhythm with all life on earth, from plants to animals, and even bacteria, and it affects more than just sleep and hunger.


1. What is a Circadian Rhythm?

Sometimes called the body clock, the circadian rhythm is about 24 hours long. All life, even the simplest cyanobacteria, has a circadian rhythm of the same duration. This daily cycle influences brain-wave activity patterns, cell repair, and hormone levels. The cycle is built into to all life, but changes in light levels and temperature conditions can alter them, if temporarily.

circadian rhythm cycle LittleBee80 / Getty Images

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