Drowsy Driving.... Almost as Bad as Drunk Driving!

Wooooo, I've driven so much lately, and it seems like every time I set foot in my car I instantly want to fall asleep. It's so dangerous! I've never had such a hard time keeping my eyes open while driving. It scares me.... I think stuff like, "I can shut my eyes if I just drive in a straight line...." BAD IDEA! So I adjust my seat, stretch, turn on the AC (full blast of course) and roll down the windows even though its only 15 degrees outside... rolling down the windows actually helps! I'm starting to believe what they taught me in drivers ed.... However, after I roll up the windows, I'm freezing so I turn on the heat full blast and start nodding off again... My eyes fill with water and my lids nearly shut. I start drifting into the other lane or off the road completely.
Why does driving put me to sleep?
It scares me....(yes, my driving scares me....)