What should I do to atone for the sin of sleeping through the ‘Isha prayer and waking up to the Fajr Adhan, knowing that I sometimes delay the ‘Isha prayer until just before Fajr due to fatigue, and have I missed out on something significant by committing that?

It is advisable to pray the 'Isha prayer before midnight, as some scholars consider it a missed prayer (qada') if performed afterwards, or its delay without a valid excuse as sinful according to others. It is disliked to sleep before the 'Isha prayer. As for making up the 'Isha prayer with the Fajr prayer, it is obligatory, and there is no sin if sleep overcame you, based on the Prophet's (peace be upon him) saying: "There is no negligence in sleep." However, if you slept after the time for the prayer had begun, and it is your habit not to wake up until after Fajr, then you have sinned, and you must repent and make up the prayer. Similarly, if you slept after the prayer time began and did not wake up until after midnight, you are considered sinful by those who deem it impermissible to delay it without an excuse. Therefore, be keen to pray the 'Isha prayer before its preferred time expires.