The reason driving while drunk is against the law is that alcohol impairs one's ability to drive safely. Voluntarily impairing your ability to drive, and then getting behind the wheel, is the reason for the law.

A person who gets behind the wheel impaired by lack of sleep is no less dangerous. The problem, I suspect, is one of proof. It's fairly easy to detect alcohol and presume impairment; it's tougher to prove sleepiness. Morally, however, the person who drives while impaired by lack of sleep is exactly as culpable as one who drives while impaired by alcohol.