Sleep

We sleep because... Yet it scarcely explains it; the majority of us assume that it's the body's natural way of relaxing after a long day. No matter how much physical or mental effort we put in during the day—or even if we spend the entire day doing nothing but sitting on the sofa—we must sleep for the same length of time. According to evolutionary theory, it would have been impossible for us to survive as hunter-gatherers for eight hours or more each day; therefore, whatever is happening inside the body during that period must be of utmost importance.

Although there are many possibilities, there is still no agreement among scientists as to why humans even sleep at all. Although studies show that the longer you stay awake, the more active your brain is when you finally fall asleep, one of the reasons is that it's an energy-saving state with little activity that allows the brain to recover. There are also all the dreams that can only occur during REM sleep, which gets shorter as you get older. Even though it is slightly more active than it is when we are awake, the brain is scarcely active during that period.