Question: Why do you get sleepy, what causes sleep?

Sam Geen answered on 21 Jun 2013:

I don’t think anyone knows why we need sleep. One idea is that the body needs time to rest and repair itself (which is why sleep is good if you’re ill; the body needs energy to fight the illness). Another is that dreaming is useful because it helps the brain reorganise itself (although this is not known for sure, and there are many ideas about why we need to dream). If we don’t sleep, it can even kill you after a while. People need to sleep for at least twenty minutes at a time to allow us to enter deep sleep. There are some people who train themselves to sleep for short periods of time throughout the day, which gives them a lot of time to do things when everyone else is asleep. This is hard to do, though, and is difficult if you need to be awake for school or work.

There’s a part of the brain called the hypothalamus that determines when we get tired – people get jetlagged because the hypothalamus depends on light levels and gets confused that the Sun is in the sky at the wrong time!