I have made some long hauls driving cross country, both for work and pleasure, and I have just never allowed myself to continue driving when I feel overwhelmed by sleepiness. I have many times pulled over and catnapped for 15 or 20 minutes at a time to refresh a little. It is just too much of a risk otherwise.
I know that with bus drivers they're on a schedule they have to keep, but if am not mistaken they are under certain restrictions about driving while overly fatigued, much the same as truck drivers are. However, I expect that much like truck drivers, they are subject to cheating and pushing the limits in many cases.
About a year ago, a church bus driver in New Orleans had an accident with a bus load, lots of injuries. It turned out he failed the drug test, apparently he smoked a joint just before picking them up.
Well I need to head out again .. but will pop back later