Sleep -like eating, drinking, and breathing- is a basic human need. Every night we experience four or five sleep cycles, which are made up of four different stages. Each stage is quite different and is designed to maintain bodily well-being and mental health.
When a child or a teenager is in deep sleep, their body produces growth hormones, so maybe there is some truth in the old saying "Early to bed and you'll grow tall". In fact, some newborn babies spend as much as 80% of their time asleep.
Deep sleep is also the time when the body mends its cells and makes the immune system stronger. If you wake up a lot at night and disturb your sleep cycle, you'll get ill more easily.
If you live an average lifetime, you'll spend about twenty years asleep.
A person kept awake for long periods usually dies after ten days of total sleep deprivation. So, if you didn't sleep for a few days, you certainly wouldn't feel good. You'd start seeing things that weren't there, and you wouldn't be able to do simple things like counting from 1 to 100.