Know your circadian rhythm, you have general two periods during a 24 hour time that you are especially tired...mine are at 1:30pm and 4-4:30am. Plan for these two times and pull over and take a power nap 20-60 min and you will feel like you slept for hours.
I tend to get sleepy about 45 minutes in to a long drive and if I take a 5-15min nap at that time, I can then drive for hours.
Also check out about using some good quality rhodiola rosa
Rhodiola rosea - Wikipedia, the free encyclopedia