Narrated by 'Abdullah bin 'Umar r.a.: that the Prophet s.a.w. said that "The time of the 'Isha' ( night ) prayer is up to midnight " [ Reported by Muslim ]
After 'Isha' prayer, it has been prevented to talk. The motive behind this prevention is that the sins of the believer are pardoned after his offering of the prayer, and it is better to go to bed than to talk and indulge oneself in worldly affairs. After offering 'Isha' prayer, going to bed earns double benefit; first, to save oneself from sins, and secondly, that the prayer will be regarded his last action before sleep.
( Reference Bulugh Al-Maram, The Book of Prayer, no. 129 &130, Soheh Muslim - The Times of Solat )