Sleep replenishes cells
WHEN you are asleep the body cells that have worked hard all day and become worn out are repaired, replaced, and build up fresh supplies of energy for the next day.
This is also when you grow the most. Your brain also takes a rest, but is active enough to keep you alive, in other words, you continue to breathe and your heart continues to beat.
But with your eyes closed, your brain does not receive the signals it has to decipher when you are awake. - bigsiteofamazingfacts.com