Nap is the safest option. If I wasn't too tired where I was falling asleep, but just tired of driving and getting a little road daze, I used to change volume on the stereo every 5-10 minutes, change the temperature, either by blasting the air, or rolling up/down the windows every 10-15 minutes. Changing positions while sitting, and playing little games with other cars, usually the big semi's - where I would let them get far away, and then catch up, maybe pass, and repeat.