Asleep at the Wheel

For some reason, I have been getting tired when I am getting ready to drive somewhere, usually a few minutes after I start driving. The weirdest part is that up until I start driving, I am full of energy, but for some reason, when I start to drive, I get sleepy. I have heard several stories about fatal car accidents because people fell asleep at the wheel. I am worried that one day, I really will fall asleep at the wheel, so I try to sleep enough and stay awake when I need to be.