When has one missed Isha prayer?

Bismillahir-Rahman-ir-Rahim Assalaamu 'Alaykum ya Shayukh If I fall asleep before Salaat-ul-Isha, and wake up after midnight, when I make Salaat-ul-Isha, is it Qadha because it is in the time of Salaat-ul-Tahajud, or is it still within the allowed time for Isha? 

Walaikum assalam wa rahmatullahi wa barakatuh,

I pray this finds you in the best of health and spirits.

The time of Isha lasts until Fajr enters. Thus, if you wake up late in the night, your Isha prayer would still be considered a current performance (ada') and not a make up (qada'). However, it is somewhat disliked (makruh tanzihan) to sleep before performing Isha if there is any possibility of missing it, or when it will mean performing it in the second half of the night--unless there is a reason or excuse (such as an awkward work schedule).

[ Ibn Abidin, Radd al-Muhtar; Tahtawi/Shurunbulali, Hashiyat Maraqi al-Falah ]

And Allah alone gives success.

Faraz Rabbani