Because if they made it manditory all people would do is gripe.

There are devices out there that truckers use so that when your head bobs it gives a tingling "shock" to your neck.

Personally, I've stopped at 4:00 in the afternoon and took a 15 minute nap if I was that bad. It makes a big difference. Once I get past that 3-6:00 PM I'm usually good to go until late that night.