Praise be to Allah.
It is permissible to delay ghusl until the time of Fajr but it is mustahabb (recommended) for them to do wudoo’ before they sleep. This was the view of the majority of scholars, and al-Nawawi reported that there was scholarly consensus on this point.
(al-Kaafi, 1/173; al-Muhadhdhab, 1/33; al-Mughni, 1/229)
Their evidence is the hadeeth of ‘Aa’ishah (may Allaah be pleased with her) whose authenticity is agreed upon, in which she narrated that when the Messenger of Allaah (peace and blessings of Allaah be upon him) wanted to sleep whilst he was in a state of impurity (junub), he would wash his private parts and do wudoo’ as for prayer.
(Narrated by al-Jamaa’ah). They also use as evidence the report narrated by Ibn ‘Umar (may Allaah be pleased with him) who said that ‘Umar said, “O Messenger of Allaah, may anyone of us sleep when he is in a state of impurity?” He said, “yes, if he does wudoo’.” (Agreed upon).
The wisdom behind doing wudoo’ is that it reduces the impurity and is better for the body.
The Zaahiriyyah and Ibn Habeeb among the Maalikis thought that it was obligatory for the person who is in a state of impurity to do wudoo’, on the basis of the hadeeth of ‘Umar referred to above. And Allaah knows best.