Thank you. It seems a bit random if you wake up at 6 or 7; I apparently received a string of days waking up at 7, especially when going to the nightclub.
Haven't played the game in a while and have no idea if that's the way it works, but I had the impression that:
The game tries to let the MC sleep "enough" or until he has to get up (and considering you always have to eat breakfast at 8 - even on the weekend - the game gives you enough time to properly have time beforehand) and wakes you up at 7 am.
So if you go to sleep at midnight or 1 am, you wake up the next day at 6.
But if you go to sleep at 2 am, 3 am or an activity that the game says ends after 1 am...the game lets you sleep to 7.