Why am I so sleepy?!

I sleep a fairly healthy amount. I usually get 7-8 hours a night and I'm fairly active throughout the day. The problem is about an hour or two after waking up, even after I've eaten a nutritious breakfast, I feel sleepy and usually end up falling asleep and taking a nap that can last up to three hours! My dad suggested that I might have a vitamin deficiency, but I'm not sure. I'm not narcoleptic. I do have the sleepless night here and there, but it's been a LONG time since that's happened.