Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.088.

One should not sleep before the night prayer, nor have discussions after it. Abu Barza al-Aslami related that the Prophet, upon whom be peace, loved to delay the night prayer (which was called darkness, al-‘atmah) and he hated sleeping before it and talking or discussions after it.” (Related by “the group.”) In another saying by Ibn Mas’ud, it is reported, “The Messenger of Allah ordered us not to talk after the night prayer.” (Related by Ibn Majah.)

The reasons behind this are: sleep may make a person miss the night prayer in its best time, or it may cause him to miss the congregational prayer, and talking and socializing afterwards would cause one to misappropriate a time from which he could greatly benefit. If one wants to sleep and has someone to wake him up, or he is discussing a beneficial matter, then it is not disliked. Said Ibn ‘Umar, “The Prophet would discuss with Abu Bakr some of the affairs of the Muslims during the night, and I was with him.” (Related by Ahmad and at-Tirmizhi, who said it is hassan.) Reported Ibn ‘Abbas, “I slept in the home of Maimunah one night when the Prophet, upon whom be peace, was there. I watched to see how the Prophet prayed during the night. He talked with his wife for a while and then slept.” (Related by Muslim.)