i usually go to bed at 4am. i went to bed about 3 1/2 hours earlier than that last night.
i woke up at 7. and freaked the fuck out because it was dark outside and i thought i had slept through my class. it was a complete mindfuck and i haven't recovered.
....and that is why i stay up until 4am. then i'm guaranteed that the first time i wake up, it'll be daylight and i won't freak out about sleeping all day.
I did that a couple of weeks ago. Fell asleep in the afternoon, got up to go to my lecture and realised something funny was going on when I realised it was starting to get dark and there were rather a lot of people about in the JCR and the such. Very disorientating when you first wake up too.