Chronic lack of sleep has been linked to high blood pressure, diabetes, aging skin, elevated cholesterol levels, weight gain and more. Now researchers have done a study that seems to suggest missing shuteye has another less serious side effect: the inability to tell when someone else is happy or sad by looking at his or her face.

In the small study – which was led by William Killgore, a University of Arizona professor of psychiatry, psychology and medical imaging – 54 participants were shown 180 photos that were created by morphing commonly confused facial expressions, such as disgust and anger. The participants were asked to identify the emotions they believed each image represented.

Then the study volunteers were deprived of sleep for a full night and asked to repeat the test.

The researchers found that, while everyone could still identify obvious facial expressions like a big smile or frown after a sleepless night, they had trouble identifying the more subtle expressions of happiness and sadness, such as an image that might show 70 percent sadness and 30 percent disgust.

A sample of the way expressions were computer blended for the study (Credit: University of Arizona)

Interestingly, when it came to other emotions like anger, fear, surprise and disgust, there was no change in correct comprehension regardless of sleep levels. The researchers posit this may be because happiness and sadness don't pose an immediate threat to our wellbeing when expressed by someone else, so when we're fatigued, we instinctively know it's OK to drop the processing of those emotions on the faces of others.

"If someone is going to hurt you, even when you're sleep deprived you should still be able to pick up on that," Killgore said. "Reading whether somebody is sad or not is really not that important in that acute danger situation, so if anything is going to start to degrade with lack of sleep it might be the ability to recognize those social emotions."

Killgore also says that in a chronically overtired world, missing the facial cues of others could have a fairly significant societal impact.

"As a society, we don't get the full seven to eight hours of sleep that people probably need to be getting," he said. "The average American is getting a little less than six hours of sleep on average, and it could affect how you're reading people in everyday interactions.

"You may be responding inappropriately to somebody that you just don't read correctly, especially those social emotions that make us human. Or you may not be as empathic. Your spouse or significant other may need something from you and you're less able to read that. It's possible that this could lead to problems in your relationships or problems at work. To me, that is one of the biggest problems — how this affects our relationships."

The study has been published in the journal Neurobiology of Sleep and Circadian Rhythms.

Source: University of Arizona

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