We all know that a lack of sleep can leave us feeling bleary-eyed and cranky, but researchers in California have shown that sleepy people also have trouble reading facial expressions, struggling to distinguish threatening faces from friendly ones.
"Recognising 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 Matthew Walker, professor of psychology and neuroscience at the University of California, Berkeley.
In this experiment, 18 volunteers viewed a variety of facial expressions, once after a full night’s sleep and once after 24 hours of being awake. As the participants viewed the expressions, they each had their brain scanned using functional magnetic resonance imaging (fMRI) and their heart rate monitored.
The brain scans revealed that the emotion-sensing areas of the sleep-deprived brain were unable to distinguish between the friendly and threatening faces. The participants’ heart rates also failed to respond to the expressions. What’s more, the researchers found that the neural link between the brain and the heart – typically used by the body to send distress signals – was disconnected.
As a result, the volunteers who hadn’t slept were more likely to interpret friendly or neutral faces as menacing.
"Insufficient sleep removes the rose tint to our emotional world, causing an overestimation of threat,” says Walker. “This may explain why people who report getting too little sleep are less social and more lonely."
These findings highlight just how important sleep is in all aspects of our lives, from forming memories to realising that not everyone is out to get you!