What is the ruling on delaying the 'Isha prayer until shortly before dawn due to overwhelming sleepiness, and is that considered one of the major sins, and what is its expiation?

The Isha prayer has two times: an optional time (waqt al-ikhtiyar) which extends from the disappearance of twilight until one-third or one-half of the night, and a time of necessity (waqt al-darurah) which extends until dawn. If a person sleeps intending to wake up and pray before the optional time expires, but then oversleeps, there is no sin upon him, especially since the Sunnah is to delay Isha. However, if he sleeps after the time has entered, knowing or thinking that he will not wake up until after the optional time has expired, then he is sinful and repentance is incumbent upon him, because he delayed the prayer until the time of necessity, and this is not permissible.