I get why we need to sleep and all that. But why specifically 8 hours? Why not 4 or 13 or any other random number of hours?
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The number of hours does vary by age, as you can see [from this article](https://www.cdc.gov/sleep/about_sleep/how_much_sleep.html), and as far as why specifically 8 hours, we typically sleep at night, so that likely has something to do with it.
The why is going to be a guess as that’s asking what forces over millions of years happened to result in us sleeping about 8 hours and that’s too hard to truly determine.
The what happens that we need 8ish hours is based on the fact that we sleep in cycles. Initially we fall asleep and go through a cycle where our body just barely dips into deep sleep. This takes about 2 hours. When that cycle ends, a new one starts but this time there’s more deep sleep. This rapists about 4 times for about 8 hours of sleep with the body getting more deep sleep every cycle. That’s why it’s important to sleep for 8 hours or so at a time and not sleep six hours and then get a couple of one hour naps.
Deep sleep is important because it allows a lot of progress and parts of our body to reset/recharge. Imagine a bucket with a small hole in the bottom. Throughout the day we fill that bucket and while some of what goes in is able to get out. During sleep we’re able to keep letting the bucket empty through that hole without adding more to it. During deep sleep the hole gets larger and helps to completely empty the bucket. So, sleep is better than no sleep, but getting a solid amount all way once is even better.
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