Islam is the religion of cleanliness. Allah Almighty says, (Truly Allah loves those who turn unto Him, and loves those who have a care for cleanness.) (Al-Baqarah 2: 222) The Prophet (peace and blessings be upon him) is reported to have said, “Cleanliness is half of faith.”
Sheikh M. S. Al-Munajjid, a prominent Saudi Muslim lecturer and author, states the following: It is permissible to delay ghusl until the time of Fajr, but it is recommended for the husband and wife to perform wudu’ before they sleep. This was the view of the majority of scholars, and An-Nawawi reported that there was scholarly consensus on this point (Al-Kafi, 1:173; Al-Muhadhdhab, 1:33; Al-Mughni, 1:229).
Their evidence is the hadith of `A’ishah (may Allah be pleased with her), whose authenticity is agreed upon. She narrated that when the Messenger of Allah (peace and blessings be upon him) wanted to sleep whilst he was in a state of impurity (janabah), he would wash his private parts and perform wudu’ as for Prayer.
They also use as evidence the report narrated by Ibn `Umar (may Allah be pleased with him) who said that `Umar said, “O Messenger of Allah, may anyone of us sleep when he is in a state of impurity?” He said, “Yes, if he performs wudu’” (Reported by Al-Bukhari and Muslim).
The wisdom behind performing wudu’ is that it reduces the impurity and is better for the body.
The followers of the Zhahiri school of thought and Ibn Habib among the Malikis thought that it was obligatory for the person who is in a state of impurity to perform wudu’, on the basis of the hadith of `Umar referred to above.