Not getting enough sleep is detrimental to both your health and productivity. Yawn. We’ve heard it all before. But results from one study impress just how bad a cumulative lack of sleep can be on performance. Subjects in a lab-based sleep study who were allowed to get only six hours of sleep a night for two weeks straight functioned as poorly as those who were forced to stay awake for two days straight. The kicker is the people who slept six hours per night thought they were doing just fine.

This sleep deprivation study, published in the journal Sleep, took 48 adults and restricted their sleep to a maximum of four, six, or eight hours a night for two weeks; one unlucky subset was deprived of sleep for three days straight.

Subjects who got six hours of sleep a night for two weeks straight functioned as poorly as those who were forced to stay awake for two days straight.

During their time in the lab, the participants were tested every two hours (unless they were asleep, of course) on their cognitive performance as well as their reaction time. They also answered questions about their mood and any symptoms they were experiencing, basically, “How sleepy do you feel?”

Why Six Hours of Sleep Isn’t Enough

As you can imagine, the subjects who were allowed to sleep eight hours per night had the highest performance on average. Subjects who got only four hours a night did worse each day. The group who got six hours of sleep seemed to be holding their own, until around day 10 of the study.

In the last few days of the experiment, the subjects who were restricted to a maximum of six hours of sleep per night showed cognitive performance that was as bad as the people who weren’t allowed to sleep at all. Getting only six hours of shut-eye was as bad as not sleeping for two days straight. The group who got only four hours of sleep each night performed just as poorly, but they hit their low sooner.

The six-hour sleep group didn’t rate their sleepiness as being all that bad, even as their cognitive performance was going downhill.

One of the most alarming results from the sleep study is that the six-hour sleep group didn’t rate their sleepiness as being all that bad, even as their cognitive performance was going downhill. The no-sleep group progressively rated their sleepiness level higher and higher. By the end of the experiment, their sleepiness had jumped by two levels. But the six-hour group only jumped one level. Those findings raise the question about how people cope when they get insufficient sleep, perhaps suggesting that they’re in denial (willful or otherwise) about their present state.

We Have No Idea How Much We Sleep

Complicating matters is the fact that people are terrible at knowing how much time they actually spend asleep.

According to the Behavioral Risk Factor Surveillance System survey, as reported by the CDC, more than 35% of Americans sleep less than seven hours in a typical day. That’s one out of every three people. However, those who suffer from sleep problems don’t accurately estimate how much they sleep each night.