Abu Hurairah (Ra) narrates that Rasúlullãh (saw) said: When any one of you goes to sleep, the Shaitãn ties three knots at the back of his neck, sealing every knot with the words: You have a long night, so sleep. So if one awakens and remembers Allãh a knot will be loosened; if he performs ablution, a knot will be loosened; and if he offers Salãt, a knot will be loosened. And in the morning he will be active and in pleasant mood; and having received a great blessing. If he does not offer Tahjjud, he will be sluggish and in disturbed mood, also being deprived of a great blessing.

(Abu Dãwúd, Ibne-Mãjjah)