If you're not getting sufficient sleep, it might make you unable to differentiate between a friend and a foe based on a person's facial expressions, researchers say.

In a study at the University of California, Berkeley, scientists found that people who are sleep deprived have trouble accurately reading facial expressions such as smiles or scowls.

More than just an annoyance, it could have serious ramifications, such as an inability to notice that your child is ill or in pain, or that a person approaching you might be a potential mugger or a violent predator, the researchers point out.

"Recognizing the emotional expressions of someone else changes everything about whether or not you decide to interact with them, and in return, whether they interact with you," says Berkeley neuroscience and psychology professor Matthew Walker.

"These findings are especially worrying considering that two-thirds of people in the developed nations fail to get sufficient sleep," says Walker, senior author of the study appearing in The Journal of Neuroscience.

In an experiment, 18 healthy adult volunteers were shown images of 70 facial expressions, ranging from friendly and approachable to menacing and threatening.

One viewing was conducted after the participants had had a full night's sleep; then, it was repeated again after they had been awake for a full 24 hours.

As the volunteers viewed the images, the researchers conducted brain scans and measured heart rates.

The Magnetic Resonance Imaging scans showed that in the sleep-deprived viewing session, the participants' brains were unable to distinguish a difference between friendly and threatening faces, the researchers reported.

That was specifically seen in the brains' regions responsible for emotion-sensing, they said.

In addition, heart rates did not respond as usual when they were sleep deprived, and the neurological link between the brain and heart normally enabling the body to sense distress signals seemed to be disconnected.

"Sleep deprivation appears to dislocate the body from the brain," says Walker. "You can't follow your heart."

The inability to judge the potential behaviors implicit in facial expressions could especially impact certain sleep deprived groups, noted study lead author Andrea Goldstein-Piekarski, a postdoctoral fellow at Stanford University who began the study while a Ph.D. student at Berkeley.

"Consider the implications for students pulling all-nighters, emergency-room medical staff, military fighters in war zones and police officers on graveyard shifts," she says.

Photo: Tim Green | Flickr

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