Praise be to Allah.
‘Isha prayer must be performed before midnight, and it is not permissible to delay it until midnight, because the Prophet (peace and blessings of Allaah be upon him) said: “The time of ‘Isha’ is until midnight” (narrated by Muslim, al-Masaajid wa Mawaadi’ al-Salaah, 964). So you have to pray it before midnight, based on the length of the night, because the night may be longer or shorter, so the guideline is how many hours the night lasts. If the night is ten hours long, then it is not permissible to delay it until the end of the fifth hour. The best way it to pray it in the first third of the night. If a person prays it at the beginning of the time for ‘Isha, that is OK, but if he delays it a little while, that is preferable, because the Messenger (peace and blessings of Allaah be upon him) used to recommend delaying ‘Isha’ prayer for a little while. But if someone prays it at the beginning of its time, after the twilight – the reddish afterglow along the horizon – has vanished, there is nothing wrong with that. And Allaah knows best.