I’m still not quite sure how, when in sixth form, I used to go out 2 or 3 times a week, and if I wasn’t out I’d still stay up till about 2am most nights, then I was up at around 7 every day, and I just used to get on with it, maybe sleep a little longer at the weekends but that was it. 
Now unless I go to bed dead early or have a mega long lie in I feel wiped out all day. And if I’ve had a night out  I tend to lose the day afterwards.
I’m getting old.