Why am i always so tired?

I've got iron deficiency and polycystic ovaries, but I cant stop being so tired. I'm only 18, so people think I'm crazy when we're out and I'm standing in a club yawning! My teachers much think I'm rude because I yawn all the way through their lessons. I do go to bed late but I find that it doesn't make a difference. The other week I went to bed on Friday, got up on Sunday, and I was still tired! I don't have diabetes, my weight is normal, I only drink tea, water or juice unless I'm out... For a long time even I thought that I was just lazy, when boyfriends came round I'd fall asleep within 5 minutes of them being there then wake up to say goodbye when they left! If not for the social aspect, it's almost impossible to revise when I end up laying down, curling up, and waking up ridiculously later. Why can't I stay awake, where has my energy gone!!??