sleeping madness

hey, im kinda lost as to what is happenin, first il jus set the scene a bit.
im doin my alevels so i get a few free lessons here and there, and theres this one day every week wher i have first and third free and then its lunch, so usually i get to stay in bed for about half an hour longer b4 i make myself get up n e way.
thing is last week, i didnt wake up till 11 so id missed the one lesson i had in the morning, it rly freaked me out, but i suffer from depression and id had an unbelievably crazy night before, it was like, one of the worst nights of my life, so i sort of figured that it was just my brain recovering.
problem is, it hapened again this week, when id had a fine evening the night b4, granted i was up till 2am doing this coursework piece but tht isnt unnatural for me, it in fact tends to be when i work best!
but now im really confused, scared that it will happen next week (cos i set three alarms which i slept straight through) and angry for being so stupid.