Primarily, sleep is for the brain, allowing it to recover and regenerate. While we are asleep, the brain processes information and consolidates memory. Sleep not only strengthens memories but allows the brain to reorganise them by picking out emotional details, which in turn produce new insights and can create new ideas.
Sleep also provides rest for our bodies and allows us to regenerate and reenergise for the next day. Our immune system becomes strengthened, the body repairs and grows cells, and our sympathetic nervous system gets a break.