It is permissible to delay ghusl (ritual bath) until the time of Fajr, but it is recommended for the husband and wife to perform wudu (ablution) 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 ablution 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 ablution.” (Al-Bukhari and Muslim)
The wisdom behind performing ablution is that it reduces the impurity and is better for the body.
Allah Almighty knows best.