Question: why do we get tired?

Ciarán O'Brien answered on 19 Nov 2014:

One of the main things that makes us feel tired is adenosine. It’s a chemical that the brain produces while we’re awake, and as it builds up we feel the urge to sleep. Caffeine like you get in tea or coffee can reduce the effect of adenosine, which is why a strong cuppa can make you feel more awake.

Nobody’s fully sure why it happens yet, but scientists have been working on it for a while now and they’re finding out some interesting things that happen:

A lot of the body’s repair and growth systems work best (and sometimes only) while you’re asleep, and animals who go without sleep have way worse immune systems. Some scientists think sleeping gives the body a chance to concentrate on these functions.

Another idea is that sleep lets your brain develop and rewire itself while it’s not busy. Babies sleep around 14 hours a day, and during that time they’re in the kind of deep sleep that allows you to dream. Scientists have found that in grown ups the brain rewires itself a lot while asleep too (it’s called neuroplasticity, or just plasticity). Grown ups who don’t sleep enough have difficulty learning new things, and their memory gets worse. Sleep and dreams could be a way for the brain to sort through what it learned for the day, and make sure that the lessons stuck by rewiring itself to be better at whatever you were taught.

There could be other reasons why we sleep, but those are pretty good explanations for why we get tired. It’s because our brain needs to take a moment to rewire itself, and the body needs some downtime to grow and repair.