Hadith.

1165. Abu Hurairah (May Allah be pleased with him) reported: The Messenger of Allah (PBUH) said, "When any one of you sleeps, Satan ties three knots at the back of his neck. He recites this incantation at every knot: `You have a long night, so sleep.' If he awakes and remembers Allah, one knot is loosened. If he performs Wudu', the (second) knot is loosened; and if he performs prayer, (all) knots are loosened. He begins his morning in a happy and refreshed mood; otherwise, he gets up in bad spirits and sluggish state.'' [Al-Bukhari and Muslim]. Commentary: "Tying knots'' can be literal, the way magicians do it in their magical exercises. With this practice Satan tries to withhold people from the worship of Allah. Some people are of the opinion that it is a metaphor for sleepiness/dozing. In any case, Satan does his level best to withhold a person from the worship of Allah. Satan experiences frustration if someone gets up at night for prayer; he also experiences happiness if he manages to keep someone asleep until dawn.