How Does The Body Clock Work?

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The body clock, or circadian clock, is an internal clock that keeps track of time. Circadian comes from 'circa-diem' meaning 'around a day.' Although circadian rhythms are endogenous (built-in, self-sustained), they are adjusted to the local environment by external cues called zeitgebers, commonly the most important of which is daylight.
Last month scientists discovered proteins within our cells that prevent us from drastically adjusting our body clock, leading to the familiar sensation of jet lag.