If you struggle to wake up in time for school or work or feel drowsy during a trip abroad, your circadian rhythms may be out of sync with your environment. Circadian rhythms are your internal timekeepers, and almost all organisms, from bacteria to plants and animals, have them. You can’t see them, but you can feel their effects—they control when you get sleepy, when you wake up in the morning, and when you feel hungry. Among other signals, the brain uses sunlight to keep time.
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