Definitions of drowsy driving or driver fatigue rely on how the concept of "fatigue" is defined. Fatigue is a general term commonly used to describe the experience of being "sleepy," "tired," "drowsy," or "exhausted." While all of these terms have different meanings in research and clinical settings, they tend to be used interchangeably in the traffic safety and transportation fields.

There are many underlying causes of sleepiness, fatigue and drowsy driving. Including sleep loss from restriction or too little sleep, interruption or fragmented sleep; chronic sleep debt; circadian factors associated with driving patterns or work schedules; undiagnosed or untreated sleep disorders; time spent on a task; the use of sedating medications; and the consumption of alcohol when already tired. These factors have cumulative effects and a combination of any of these can greatly increase one's risk for a fatigue-related crash.

Sleepiness or Fatigue Causes the Following:

• Impaired reaction time, judgment and vision
• Problems with information processing and short-term memory
• Decreased performance, vigilance and motivation
• Increased moodiness and aggressive behaviors

In addition to the dangers of driving under the influence of fatigue, several states are considering legislation that would allow police to charge drowsy drivers with criminal negligence if they injure or kill someone while driving if they have not had adequate sleep. For more information about these policies, see "drowsy driving policies."