People are notoriously bad at judging their level of sleepiness and alertness (or vigilance). This lack of self awareness is the main cause of drowsy driving accidents and a reason that adolescents and young adults are more prone to drive while drowsy – they underestimate their sleepiness.
The first regulations (U.S.) on commercial drivers were formulated in 1938 when the understanding of sleep physiology was less than it is today. These regulations limited shifts to 10 hours and required an 8-hour rest period between shifts. Experts debated whether 8 hours was sufficient since some of it would not be spent sleeping. Further, longhaul drivers attempting to push the limits effectively could adopt and 18-hour day within a 24-hour period and almost cheat the intent of the law. Indeed, truck stops adjusted to the rules and stayed open 24 hours per day and offered sleeping accommodations at odd hours. There were also no federal restrictions on drivers with sleep disorders.
The regulatory environment has shifted from a focus on just safety to include a need to stress increased efficacy. Today the federal Department of Transportation regulates interstate truck driving, and state transportation agencies have more or less adopted the same guidelines and rules.
Everybody wants to reduce drowsy driving, but it is a difficult problem to solve. The U.S. government'a Healthy People initiative seeks to reduce the crash rate from 2.7 per 100 million miles driven (their estimate for 2009) to 2.1 per 100 million miles driven by 2020.
The National Conference of State Legislatures website has a page with Summaries of Current Drowsy Driving Laws.