Yahya related to me from Malik from Zaydibn Aslam that the ayat "You who believe! When you rise for prayerwash your faces, and your arms to the elbows, and wipe over your headsand your feet up to the ankles," refers to rising from bed, meaningsleep.Yahya said that Malik said, "The situation with us isthat one does not have to do wudu for a nose-bleed, or for blood, orfor pus issuing from the body. One only has to do wudu for impuritieswhich issue from the genitals or the anus, or for sleep."
USC-MSA web (English) reference: Book 2, Hadith 10
Arabic reference: Book 2, Hadith 40