How Much Sleep Do You Really Need?

In other words, while eight hours a night may be the best option for your physical health, it's often not the best option for the health of your career. This, of course, is intuitive. "You have a deadline that needs to get done, you're going to stay up late to do it because staying up late is worth it," says Showalter. "In the real world, kids are faced with tradeoffs. And one of those is homework versus sleep or social activities versus sleep."

A catch-22.

Balancing the needs of our career with the needs of our health is complicated enough; so what happens when we don't have all of the necessary information required to make informed decisions? Is it possible to be chronically sleep deprived without realizing it?

Back in 2003, a team of researchers that included Dinges, conducted a two week experiment in which participants were divided into three groups: one got four hours of sleep a night, one got six hours a night, while a control group slept a luxurious eight hours a night. Every day each participant was given a series of psychomotor vigilance tasks (PVT), such as pressing a button when a bright spot appeared on a computer screen. On PVT tests, even slight delays in a participant's reaction time indicate lapses in alertness and wakefulness, says Dinges.

How Much Sleep Do You Really Need?
David Dinges, PhD

At the end of the experiment, participants who had logged eight hours a night displayed no impairment. But the reaction times for participants in the restricted sleep groups steadily declined over the two-week period. By day 14, the six-hour group's average score deteriorated to the point where it matched the average score for an individual who had been up for 24 hours straight.

The scary thing? Participants didn't recognize how impaired they had become.

By the end of the 14 days of sleep restriction, subjects in the four and six-hour sleep groups weren't aware that their performance had tanked. "The self-reports were all saturating exponential functions," says Dinges."They show some change after the first few days, but then they level off." In other words, while participants were able to recognize a decrease in their reaction time after a few days, eventually they began to believe they'd adapted to the restricted sleep schedule; their reaction times continued to worsen, but they were unable to recognize the continual decline.

"They'd say: 'I've adapted. I'm a little tired, but I'm alright," Dinges recalls. "That's what's so dangerous about allowing a deficit to build up. You never experience a true recovery and so the brain never resets. It begins to think the sleep deprived state is normal."

Related: Yes, You Can Sleep In and Still Be Successful

And when it comes to our ability to handle sleep deprivation, well…some of us can do it better than others. "You can have two people who need exactly the same amount of sleep a night," Dinges says. "Except one of them is extremely vulnerable to sleep loss and has cognitive deficits within one night of reduced sleep, while the other one doesn't experience cognitive deficits until five nights on a reduced sleep schedule."

This resistance, Dinges says, has nothing to do with the amount of sleep a person needs a night. You may need nine hours a night, but still be incredibly resistant to cognitive deficits that accompany sleep loss. Or you may only need six hours a night, but face extreme cognitive deficits if you try and get away with just five.

It's a catch-22 of sorts. The less you sleep and the more cognitively impaired you become, the less qualified you are to recognize that you are cognitively impaired in the first place. Suddenly, it becomes easier to buy into the fiction that fatigue can be fought with caffeine, bright lights and sheer willpower.

So how can you determine your ideal sleep schedule?

While the majority of us need somewhere between seven and nine hours, that's still a large window.

In addition, optimal sleep time – the hours at which we get our best rest – differs dramatically throughout the population. Back in 1996, a director of a sleep clinic approached Dr. Fu with a special case; one of his patients, a 69-year-old woman, exhibited an unusual sleep cycle; like clockwork, she would get incredibly tired at 8 p.m., and consistently wake up around 4 a.m. each morning.  Worried after noticing the same unusual sleep pattern in her granddaughter, the woman visited a host of doctors; many concluded that she was suffering from undiagnosed depression. Why else would she be up at 4 a.m. every morning?

Laura Entis is a staff writer at Entrepreneur.com.
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