A new study from UC Berkeley spells bad news for the sleep-deprived, who, they found, have a much harder time interpreting facial expressions. The ability to read faces is crucial for understanding others -- some neuroscientists believe that 93% of all communication is nonverbal

The study, published today in the Journal of Neuroscience, asked 18 young adults to view 70 images of faces making various expressions, once on a normal night's sleep and again after staying up for 24 hours. The researchers measured the participants' heart rates and scanned their brains while they viewed the image.

They found that, when sleep deprived, people couldn't tell the difference between a friendly and threatening face. Additionally, their heart rate did not act as normal, failing to rise with the sight of a threatening face. Heart rate is one of our body's main indications that we're in danger, so this disconnect could be very dangerous. "Sleep deprivation appears to dislocate the body from the brain," said Matthew Walker, the senior author of the study. "You can't follow your heart." The results also showed that people were more likely to see expressions as unfriendly after sleep deprivation, perhaps explaining the generally cranky demeanor of people who don't get enough sleep. 

Though the data set is small, these results are worrying. People with high-stress jobs requiring good communication are often sleep deprived. "Consider the implications for students pulling all-nighters, emergency-room medical staff, military fighters in war zones and police officers on graveyard shifts," Andrea Goldstein-Piekarski, one of the study's researchers, said. 

Cover: Flickr