Just my personal experience:
every summer, after university finals, I used to sleep at around 5 am and wake up at 1 pm. It happened this summer and in 2010.
I noticed that the it only happened when I was not active during the day. And by "active" I don't just mean having a job or just going out, but exercising 3 times a week and going for a run at least twice.
This routine isn't for everyone but if you're 18-30, this is a must for you IMO. In turn, I get 8 hrs of sleep. (and having a 8 am class helps)
When I am active, I sleep like a baby.
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