How sleep works: what pulls you to bed, and what your brain does all night
The body clock: how your brain knows it is night without a window
Stay awake through the night, and the hours do not get harder one by one: one of them is harder than all the rest. How does your brain know it is night, with no window?
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5 am. This is the hardest hour of the whole night. Rono is asleep on the board, and Miva stares at her dice, no longer sure whose turn it is.
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Part of the reason sits deep in your brain: a group of cells that keeps time, your body clock. At this hour it says "deep night", and it turns your body down. Even your temperature is near its lowest of the day.
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This clock keeps running even with no window. In 1962, Michel Siffre lived two months in a cave, with no watch and no daylight. He still slept and woke on a cycle of a bit more than 24 hours.
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A bit more than 24 hours means a clock that slowly runs late. Daylight puts it back on time: light that reaches your eyes in the morning tells the clock that the day starts now. This daily rhythm of about 24 hours is your circadian rhythm.
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5
Outside the hall, the sky starts to turn grey. Your body clock is about to get its signal.
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Quick check
You fly to a city 6 hours ahead of home. At 11 pm there, you feel wide awake. Why, and what fixes it?
Show the answer
Your body clock still runs on home time, where it is only 5 pm. Daylight in the new city moves it a little each day, until it matches.
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Go further
Three more things about the clock.