Reference: ‘Ishratun-Nisaa. wa Hill al-Khilaafaat az-Zowjiyyah – Page 59
Question: Is it permissible for one who is junub to sleep before making wudhoo?
Response: There is no sin upon him if he slept without making wudhoo. However, it is better that he makes wudhoo before he sleeps, as the Prophet (sal-Allaahu ‘alayhi wa sallam) did this and encouraged it.
And with Allaah lies all success and may Allaah send prayers and salutations upon our Prophet (sal-Allaahu ‘alayhi wa sallam) and his family and his companions.