A UC Berkeley study shows that sleep deprivation dulls our ability to accurately read facial expressions. This deficit can have serious consequences, such as not noticing that a child is sick or in pain, or that a potential mugger or violent predator is approaching. 18 healthy young adults viewed 70 facial expressions that ranged from friendly to threatening, once after a full night of sleep, and once after 24 hours of being awake. Researchers scanned participants’ brains and measured their heart rates as they looked at the series of visages.

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. These findings are especially worrying considering that two-thirds of people in the developed nations fail to get sufficient sleep…The better the quality of dream sleep, the more accurate the brain and body was at differentiating between facial expressions. Dream sleep appears to reset the magnetic north of our emotional compass. This study provides yet more proof of our essential need for sleep.”