Operating a motor vehicle is a complex psychomotor task that requires sustained vigilance. When sleepy, subjects have difficulty maintaining vigilance. As a result, inattention increases the risk of a motor vehicle collision. There is extensive literature that demonstrates that sleepy patients have poor performance and increased motor vehicle crashes. Determining an individual's risk of crash has not been possible solely on clinical grounds and as a result, a number of driving simulators have been developed/employed to aid in this process. While the data shows a consistent effect of sleepiness on driving and simulated driving performance, the role of these simulators in day to day clinical practice is still evolving.