I have a similar problem. My body seems to naturally want to go to sleep at 7am, and wake up at 3pm.
I've tried taking valerian (herbal sleep remedy - You can get it at most pharmacies) and it never seems to do the trick, even if I take three times as much as I should be doing.
Really, the only thing to do is to try and give yourself a routine, so that you can get used to a normal sleep pattern.
As for getting into a normal sleep pattern, I wait until the early hours, until I'm about to nod off, and hit the coffee. And I drink a few coffees in a row, and sporadically throughout the day to keep me awake, so that by the time the evening rolls around, I'llbe shattered, and drop off. I'll set my alarm for early next morning, so I don't sleep in.
I'mglad to see other people who have this problem. I've had this for years, and my dad used to give me no end of crap about it. it's bad enough being up at night with nothing to do, wishing you'd just fall asleep, but to have parents shouting at you because you have a sleeping problem only makes you want to keep sleeping during the day,and beinga wake at night when the 'rents aren't around.
oh, and another thing, the problem is much worse in the winter. A few years ago, I got into this cycle during winter, when the days are short, for about two weeks, and i can testify that seeing daylight for the first time in two weeks is an absolutely amazing experience. It's like going to heaven!