When you are behind the wheel, being sleepy is dangerous.
Sleepiness slows reaction time, decreases awareness, and impairs judgment.
Sleepiness can contribute to a crash. Everybody has a biological clock. It tells you when you're hungry, when you're most energetic, and when you're tired. It causes fluctuations in body temperature. Although there are individual differences, most people's clocks run on a daily rhythm of approximately 24 hours.