Extreme Sleep Patterns

I have the uncanny inability to sleep properly. From the start of December to last week I was sleeping 2 or 3 hours a night and I was fine off that. I'd go to bed at 3/4am and wake up between 5 and 7 am. Now, in the space of a week I have shifted to the polar opposite, pretty much. I go to bed between 7 and 10 am and wake up between 7 and 10 pm. So my question is this; why can I not sleep properly?

There is not one constant difference between now and then. I had previously thought that I have a seasonal affective disorder, as last winter I was sleeping on average 20 hours a day, and as much as 30 hours in one go. Although that is an extreme example as I was also ill, it was a pattern for a few years. Now I got to France and couldn't sleep in the winter, but now I'm back to the general trend. WTF is wrong?

I am pretty sure that when it gets to the summer I'll sleep less again, and I just can't find out why. Any suggestions?

Again with an entirely random link, a scene from Pulp Fiction: http://www.youtube.com/watch?v=SLtwFugudZE

I think I'll probably watch the film this evening, it's been a while.