Working an overnight job that's an hour away?

Sounds like when he IS sleeping, it's not restful sleep. Does he snore? Need naps? If so, he may have sleep apnea. One of the major signs is falling asleep at the wheel, more times than once. If you have insurance, go see his doc and tell him the problem. If he does have sleep apnea and starts using a CPAP, he will feel so much better and you will too knowing that he is going to work more rested.