Why do we need sleep?

We spend about a third of our lives asleep. For most of history, people thought sleep was simply rest. It is much busier than that.

While you sleep, your brain washes itself. A clear fluid flows through it and carries away waste that builds up during the day. This cleaning mostly happens in deep sleep.

Sleep also fixes memory. Things you learned in the day are replayed and stored. Students who sleep well after studying remember more than students who study longer and sleep less.

The body repairs itself too: muscles grow, the immune system makes new defenders, and hormones that control hunger are reset. One short night makes you crave sugar and snap at people. A week of short nights weakens your defences against illness. Sleep is not lost time; it is maintenance.