Explainer: Our bodies’ internal clocks

Biological clocks determine hunger, sleepiness and other daily rhythms

By Amanda Leigh Mascarelli, 11:13 AM June 14, 2013


This illustration shows how light that enters the eyes can send a signal to make sure the body’s master clock is reset daily. This ensures the internal pacemaker is always working in sync with the local time. Credit: Nat.l Inst. of Gen’l Medical Sciences



People are precisely tuned to eat, sleep and wake at specific times. These predictable patterns are known as circadian rhythms. (In Latin, circa means “around,” and dian relates to “day.”)


Circadian rhythms are biological cycles that occur about once every 24 hours. Driven by an internal “clock,” these include waking and falling asleep. But outside factors can influence the cycles too. Among such factors: diet, stress and exposure to light.


Every living thing possesses an internal master c...

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