For the song "People Are Strange," certified creepy dude and Doors lyricist Jim Morrison wrote that "faces look ugly, when you're alone." Though the band's frontman was musing on his experience of foreignness in a foreign country, his words might as well pertain to the experience of being overtired, a state that renders the familiar strange and threatening. That's according to a new Cal Berkeley study reported on by CBS SFwhich finds that a lack of sufficient sleep causes even friendly faces to appear foreign and menacing.

Rather, in the scientific language of the paper published in the Journal of Neuroscience, "Sleep Deprivation Impairs the Human Central and Peripheral Nervous System Discrimination of Social Threat."

"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,” the study's senior author Matthew Walker, a Berkeley professor of psychology and neuroscience, told the University. "These findings are especially worrying considering that two-thirds of people in the developed nations fail to get sufficient sleep,” Walker concluded.

18 healthy young adults examined 70 facial expressions ranging from friendly to threatening, the first time after a full night of sleep and the second after 24 hours of being awake. While they did so, researchers performed fMRI scans of the brains of participants as well as measured their heart rates. That "revealed that the sleep-deprived brains could not distinguish between threatening and friendly faces, specifically in the emotion-sensing regions of the brain’s anterior insula and anterior cingulate cortex."

“They failed our emotional Rorschach test,” Walker said. “Insufficient sleep removes the rose tint to our emotional world, causing an overestimation of threat. This may explain why people who report getting too little sleep are less social and more lonely.”

On the other hand, a full night of sleep, especially with sufficient REM activity, led to strong accuracy when reading faces. “The better the quality of dream sleep, the more accurate the brain and body was at differentiating between facial expressions,” Walker said. “Dream sleep appears to reset the magnetic north of our emotional compass. This study provides yet more proof of our essential need for sleep.”

Related: SF Sleeps Least Of All US Cities According To Study