Why am I having these seizures?

I have been having seizures lately, but none of them happen while I'm actually awake. For instance, I'll be pretty much asleep, in that sort of grey period before I actually fall asleep, and I'll seize up, and it'll last anywhere from a couple of seconds to five minutes. This also happens If I fall asleep during the day or when I first wake up. I was wondering why I could be having these seizures, and what could cause them.