Is it obligatory for a couple to perform ghusl immediately after intercourse, or is it permissible to delay it until waking up?

It is permissible to delay ghusl until Fajr, and it is recommended for a person in a state of janabah to perform wudu before sleeping. This is the view of the majority of scholars, and Al-Nawawi transmitted a consensus on it, citing as evidence the hadith of Aisha (may Allah be pleased with her) that the Prophet (peace and blessings of Allah be upon him), when he intended to sleep while in a state of janabah, would wash his private parts and perform wudu as he would for prayer. They also cite the hadith of Umar (may Allah be pleased with him): "Yes, if he performs wudu."

The wisdom behind performing wudu is that it lightens the state of ritual impurity and invigorates the body.

The Zahiris and Ibn Habib from the Malikis held the view that wudu is obligatory for a person in a state of janabah.