Well at uni most social events are late at night so you're going out late and then having lie ins to catch up on sleep. Then when it gets to midnight you feel like you haven't done anything with your day so you stay up a few more hours. Then you have another lie in and the cycle continues.
For most people their lecture attendance isn't monitored so people just get a bit lazy and don't wake up as early as they should. People get into bad routines.
Also of course some people don't have early morning lectures so it allows them to go to bed later.
Basically yes you have free time but most people don't use that time to sleep, they use it for socialising or staying up and watching tv. When you have so much freedom you can just do what you want and no one judges you for it at uni like your parents do.
My sleeping pattern is at its worst when I have deadlines and exams. I'm more productive at night when everyone's asleep so I tend to stay up til 6/7. Then I end up skipping lectures and sleeping in til 11. Cycle continues and it's really hard to get out of even when you no longer have deadlines.
Depends though, if you've always had a good sleeping pattern then you'll probably be able to maintain it. But if you're someone who tends to struggle, your pattern will most likely get worse at uni.