As said by sat_chit in his post, I find night driving less stressful and less tiresome. Most of my highway drives involves atleast 5-7 hours of night driving. As said, eating less for dinner is essential. I used to carry a can of redbull and it kept me awake. But from the last 5-6 trips, redbull does nothing to me. I am not sure why its so. But the best and cheaper thing I do now is carry a flask with black coffee, chocolate. I find it very effective. You have your energizer anytime you want.