It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that he (peace and blessings be upon him) said: "Satan ties three knots at the nape of your head when you sleep. After tying each knot he says: 'The night is very long, so sleep.' When the servant wakes up and remembers Allah, one knot is untied. If he performs ablution, the second knot is untied as well, and if he prays, the third knot is untied too. Then he begins the morning energetic and in good spirits. If he does not do so, he begins the morning sluggish and in bad spirits."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4727