I usually go to bed around 5ish (not tonight though...need to be up at 5 so there's really no point in sleeping), so I occupy myself with the interwebs.
Why? For one, I can't stand mornings...long ongoing period of nothing, it's a waste of my time. A lot of the people I know online are in other timezones, so it's better socially to be up at night. Also, I get privacy and relative freedom at night, and can do stuff without working around other people.
Also, on nights where I need to get work done (assignments and whatnot) I'm already accustomed to the lateness and can get a lot more work done than other people in the same situation.
...that makes me sound all lonely and anti-social, but it's not like I don't have other friends that stay up late (not AS late though), and most things get planned for the afternoon or night anyway. Downside is I DO miss out on doing day-specific things a lot since the stores around here close ridiculously early and I do have stretches of getting 4 hours of sleep per week when school/uni gets involved. But oh well.