Why do I suddenly feel sleepy?

Sometimes out of the blue and at the most inconvenient times I suddenly feel sleepy and have this sudden urge to sleep. Like sometimes in the middle of classes I feel like that, sometimes in the middle of Exams. I remember one Exam I felt this sudden urge to sleep and I tried to resist it but a couple of minutes later I woke up with my head on the desk! And I'm currently working for the Summer Vacation and a few times I feel like that. Is it because I don't get enough sleep? I usually get six hours sleep every night and I'm a twenty three year old female.