You may be at increased risk of becoming drowsy behind the wheel if you are:

• Lacking sleep or driving after extended wakefulness
• Driving during the body’s natural “low” times
• A shift worker
• All of the above
  Explanation:

  "Many drivers are at increased risk of becoming drowsy behind the wheel, including: Drivers lacking sleep; Drivers on the road during the body’s natural “low” times; Shift workers."