How Much Sleep Do You Need to Drive Safely?

How much sleep deprivation is too much before motorists no longer can drive safely?

That’s essentially the question the nonprofit National Sleep Foundation (NSF) recently set out to answer by convening a panel of scientists from multiple disciplines, including medicine and transportation.

Their consensus? People who have slept for two hours or less in the prior 24 hours are unfit to drive.

Dr. Charles Czeisler, chairman of the NSF panel and professor of sleep medicine at Harvard Medical School, calls sleep “a fundamental physiological need that no human can avoid.”

He explains:

“While individual sleep needs vary, and stimulants like caffeine may trick sleep-deprived people into feeling alert, the reality is that people are definitely impaired when they have obtained two hours of sleep or less per day.

“Though many are impaired with more than twice as much sleep, at a minimum, our two-hour threshold should serve as a red flag warning for individuals and as a guide for public policy makers.”

David Cloud, chief executive of the NSF, likens drowsy driving to distracted and drunken driving. He notes that drowsy driving causes more than 6,000 preventable deaths each year:

“We hope the work of the scientific community will inspire public policy makers, corporations and drivers to take action.”

However, the question of how a lack of sleep impacts alertness can also be complex.

Dr. Stuart Quan, a professor of sleep medicine at Harvard Medical School, explains in the school’s blog this week that several factors besides sleep duration affect a person’s level of sleepiness.

They include:

• A large amount of pre-existing “sleep debt.”
• The overall quality of a person’s sleep.
• The time of day. At night, the body’s natural clock, or circadian rhythm, is set for sleep. Therefore, people tend to be less alert at that time. This helps explain why drowsy driving accidents are more likely to happen at night, Quan says.

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