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truffle-tots t1_izrv0gi wrote

Your body tends to run in circadian (24 hour) and ultradian (90 minute) rhythms/cycles.

There is a bundle of cells known as the suprachiasmatic nucleus that receives light information from the retina. It times those rhythms with sunrise and sunset.

This allows for the proper timing for countless processes within your body.

Cyclic temperature regulation is a good example; in the morning your body cools and in the evening your core body temperature is higher, aiding relaxation and your ability to fall asleep as well as many other things. This is done via these types of rhythmic cycles via the hypothalamus which receives it's timing info from that nucleus (the nucleus is actually a part of the hypothalamus).

REM sleep cycles are another that run on these ultradian rhythms.

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