Well i googled and noone knows the exact answer but there are a few hypothesis.
The first is that sleeping allows the body to repair cells damaged by metabolic byproducts called free radicals.
the second one
Another idea is that sleep helps replenish fuel, which is burned while awake. One possible fuel is ATP, the all-purpose energy-carrying molecule, which creates an end product called adenosine when burned. So when ATP is low, adenosine is high, which tells the body that it’s time to sleep. While a postdoc at Harvard, Prober helped lead some experiments in which zebrafish were given drugs that prevented adenosine from latching onto receptor molecules, causing the fish to sleep less. But when given drugs with the opposite effect, they slept more. He has since expanded on these studies at Caltech.
Then thers the theory that the brain does a little housekeeping during sleep and cleans out the superfluous synapses
Or the brain is replying past events reinforcing memory and learning
all of these ideas have some testing behind them
Christianity, One woman's lie about an affair that got seriously out of hand
Jesus was a dead jew on a stick nothing more