For most of us, skimping on sleep means feeling cranky and foggyheaded the next day, but we all have that one friend who claims she can survive—even thrive!—on just a few hours a night.

Now, a new study suggests that people like that may indeed function better on less sleep—but they also may be more tired than they realize. In fact, they may be falling asleep throughout the day without even knowing it.

The research, conducted by University of Utah psychologists, radiologists, and neurologists, looked at MRI scans of about 900 people. The participants were split into two groups: those who reported getting a normal amount of sleep in the last month, and those who got six hours or less a night. People in the short-sleep group were then divided further, based on whether they reported daytime dysfunction—such as feeling too drowsy to perform common tasks—or said they generally felt fine.

The researchers saw something interesting in the brain scans of short sleepers that they didn’t see in the “normal” group: During their time in the MRI, their brain waves exhibited patterns more typically of sleep than of wakefulness.

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In other words, the scans suggest that some short sleepers may have briefly drifted off—even though they were instructed to stay awake, said co-author Jeff Anderson, MD, PhD, an associate professor of radiology and imaging sciences, in a press release. These patterns were seen in both sub-groups of short sleepers, regardless of whether they reported suffering daytime dysfunction.

“People are notoriously poor at knowing whether they’ve fallen asleep for a minute or two,” Dr. Anderson pointed out.

The researchers think that people who regularly get by on less sleep may have brains with wake-up systems that are perpetually in overdrive, said co-author Christopher Jones, MD, PhD, a clinical professor of neurology. “This leaves open the possibility that, in a boring fMRI scanner they have nothing to do to keep them awake and thus fall asleep,” he said.

Of course, this could lead to situations much more dangerous than conking out during a scientific study. “Other boring situations, like driving an automobile at night without adequate visual or auditory stimulation, may also put short sleepers at risk of drowsiness or even falling asleep behind the wheel,” said co-author and psychology graduate student Brian Curtis.

There was some good news for short sleepers, though. Those who said they felt fine on shorter sleep schedules also had brain scans that showed enhanced connectivity between parts of the brain associated with external sensory information and memory.

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“That’s tantalizing because it suggests that maybe one of the things the short sleepers are doing in the scanner is performing memory consolidation more efficiently than non-short sleepers,” Dr. Anderson said. (Memory consolidation, simply put, is a process in the brain that transforms short-term memories into long-term ones.) If these people are really able to consolidate their memories and brain tasks throughout the day, he explained, perhaps they actually don’t need as much sleep at night. “Maybe some brains are able to do what sleep does in little tiny epochs during the day.”

The study was published last week in the journal Brain and Behavior. More research is needed, the authors say, to determine if either of their hypotheses about the brain activity of short sleepers—or both—hold true.

The team’s next study will directly examine cognitive performance, including driving simulator testing, of people who say they need less than six hours of sleep a night. It will also include feedback from study participants’ family members and partners.

“We are particularly interested in understanding the discrepancy between people’s perception of their functioning and how they’re actually functioning,” said co-author and associate professor of psychology Paula Williams, PhD. “Not everyone is equally accurate.”