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: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.
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.