(An-Naba') 78 : 9
وَجَعَلْنَا نَوْمَكُمْ سُبَاتًا
And didn’t We make your sleep for rest?
As explained in verse 39: 42, sleep is a condition in which the soul is withdrawn for a while and life remains in the body; one loses consciousness temporarily and thus gains complete rest without pain, thought, sadness, hunger, thirst, or fatigue. That is why sleep is called complete rest. See explanation 6: 60-61.