eli5 Circadian Rhythym. how do people that don’t know what time it is when they fall asleep still wake up at the same time every day?

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eli5 Circadian Rhythym. how do people that don’t know what time it is when they fall asleep still wake up at the same time every day?

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Before civilization it was just important to be up in time – you needed to wake up in time to use what daylight hours you had. The specific time of the day wasn’t as important – sure, you needed to know to some degree how late it was to plan your day, but before civilization there was simply almost no need to meet someone at a specific time when people were living in small tribes.

The circadian rhythm is driven by a hormone called melatonin. When we have little melatonin we are awake but also start producing melatonin. When we have a lot of melatonin we get tired but also start breaking down the melatonin. If left completely alone this rhythm will repeat itself roughly every 24 hour. In some people less but in most people more. But things like light and meals will cause the melatonin levels to go down in order to tune the circadian rhythm to an even 24 hours which matches the day and night cycle.

In addition to the circadian rhythm the entire brain does not fall asleep. Part of your brain never sleeps and is able to wake up other parts on demand. This is how we have been able to stay alive for millions of years without getting eaten by predators or taken by landslides even though we need sleep every night. So even when sleeping you are constantly listening out for cues that it is time to get up and might even open your eyelids to scan the environment before falling back asleep. This means that things like a clock in the room, the sound of garbage men outside, light through the window, the neighbors alarm clock, etc. can cue your brain onto the idea that it is now time to wake up.

Your body has various clocks and mechanism. So various hormones such as melatonin and just stuff like temperature control your circadian rhythm.

Basically you have biological clocks that control you.

Like mechanical clocks, it’s not perfect and can drift and get longer.

So your body uses things like getting bright light in the morning, exercise, meal times, lack of bright light in the evening, etc. to set and calibrate your circadian rhythm.