CBS Local – When we’re tired, we can certainly be a shell of ourselves. And according a new study, we’re unable to decipher non-survival-oriented facial expressions, such as those portraying happiness or sadness.

The more relatively nuanced looks that convey happy or sad, or what researchers call “prosocial emotional expressions”, are affected by sleep deprivation.

“Findings suggest that sleep deprivation adversely affects the recognition of subtle facial cues of happiness and sadness, the two emotions that are most relevant to highly evolved prosocial interpersonal interactions involving affiliation and empathy, while the recognition of other more primitive survival-oriented emotional face cues may be relatively robust against sleep loss,” the study read, which was lead by William D.S. Killgore.

There were 54 sleep deprived adults tested, and they sifted through 120 standard facial expressions across basic human feelings: happiness, surprise, fear, sadness, disgust and anger. Outside of being unable to recognize prosocial emotional expressions, the subjects’ primal, survival-oriented senses remained unchanged. Their recognition ability resumed once they caught up on sleep.

“Sleep deprivation was associated with significantly reduced accuracy for identifying the expressions of happiness and sadness in the morphed faces,” the study read. “Gender differences in accuracy were not observed and none of the other emotions showed significant changes as a function of sleep loss. Accuracy returned to baseline after recovery sleep.”

While others can typically peg out if we’re sauntering through the day with a lack of sleep, this study confirms that we can’t tell much of the emotions laid out on their face.