People who claim they can thrive on just a few hours of sleep may actually be more tired than they realize, a new study suggests.

For most people, getting less than the recommended 7 to 9 hours of sleep per night makes them feel groggy, exhausted and cranky. But for a very small number of people, sometimes called "super sleepers," getting less than 7 hours of sleep does not seem to have any effect on how they say they feel. And there is some evidence suggesting that such people may be genetically wired to function on very little sleep.

However, the new results show that these people's real-life functioning may actually be affected by the shortage of sleep even though they report feeling just fine, said study co-author Paula Williams, an associate professor of psychology at the University of Utah. [5 Things You Must Know About Sleep]

In the study, the researchers scanned the brains of 839 people while they were awake. The participants were divided into two groups: One group who reported sleeping between 7 and 12 hours per night in the past month, on average; the other group saying they slept 6 hours or less per night in the past month, on average. The researchers then divided the latter, short-sleeping group into people who said they felt too drowsy to function normally during the day because they got so little sleep, and those who said they felt fine despite getting very little sleep on a regular basis.

It turned out that many of the people who slept 6 hours or less per night showed brain patterns that are normally seen when a person is asleep and not when he or she is awake, according to the study, published Thursday (Sept. 15) in the journal Brain and Behavior. This finding may mean that some of those people, including those who denied being dysfunctional due to the small amounts of sleep they normally got, might have actually drifted off for a bit while they were having their brains scanned.

"People are notoriously poor at knowing whether they've fallen asleep for a minute or two," study co-author Jeff Anderson, an associate professor of radiology at the University of Utah, said in a statement. [Top 10 Spooky Sleep Disorders]

The researchers said they think that the reason why those people may have possibly fallen asleep during the experiment is not only that they were tired, but also that they were bored — their attention was not being stimulated while they were having their brains scanned.

Previous research that looked at the personalities of super sleepers showed that they tend to constantly seek out stimulation, which may help them "override the need for sleep," Williams told Live Science.

Such stimulation may be the reason why they do not perceive that they are actually drowsy, she added.

"But there is evidence that if you take away stimulation, they are still showing the signs of sleep deprivation," even though they have the perception that they are not sleep deprived, she said. One real-life example of the consequence of being sleep deprived in conditions without adequate stimulation may be nodding off in the middle of a boring meeting, she said.

Originally published on Live Science.