Someone asked a pious scholar whether it is mentioned in any traditions about Satan sleeping and taking rest. The learned scholar smiled and gave a very interesting reply, " If that accursed creature ever slept, I would have some respite!"

When you sleep, the accursed Satan is fully awake. He never sleeps. He always guards over men and looks for opportunities to bring harm them.

Innahu yarakum huwa wa qabeela min haisu la taraunahum

He, and his retainers, watch over you from such a place that they can see you and you can't see them