About one-third of Americans don’t get the minimum seven hours of sleep recommended by experts. And all that sleeplessness is pretty unhealthy: Studies have shown that it’s linked to more stress, higher blood pressure, and a greater chance of getting sick. On Wednesday, however, new research introduced one more reason why getting that shut-eye is imperative: People don’t want to hang out with you when you look tired.

In the journal Royal Society Open Science, a team of psychologists and neuroscientists from Stockholm University described their research showing that “sleep-restricted” people look less attractive, less healthy, and (surprise) more sleepy. Furthermore, they found that other people aren’t interested in hanging out with sleepy-looking people. This disinterest, the researchers point out, is likely the result of evolutionary adaptations that led healthy people to stay away from unhealthy-looking people — and prevented sleep-deprived people from socializing until they caught up on rest.

The researchers solidified their beauty sleep theory by photographing 25 people after sleeping well and sleeping badly. For two consecutive nights, the study participants slept for a full eight hours and then had a relaxed, neutral photograph taken. A week later, the same participants slept for only four hours each night for two consecutive nights then came in to be photographed again.

Restlessness doesn't bode well for social lives.

These photographs were then evaluated by a different group of 122 people, who didn’t know the purpose of the study. They were asked to rate, on a scale from one to seven, how much they would want to socialize with the person in the photograph and how trustworthy, attractive, healthy, and sleepy they looked.

The raters consistently rated the people in the photographs who only slept for four hours as less attractive, less healthy, and more sleepy. (There was no correlation, however, between the different photographs and how trustworthy someone seemed.)

How exactly does sleeplessness show on a face? One reason people who are tired look less healthy, the researchers explain, is that there’s a noticeable difference in the “skin blood coloration” of people who slept well and those who didn’t. A healthy, attractive face is also more red, which is a sign of increased vasodilation and vascularization; in other words, more blood is flowing through its veins. Whether you’re aware of the science of redder faces doesn’t actually matter; it’s that people inherently understand what looks healthy because evolution has ingrained the link between blood flow and health in our brains.

“Blood flow to the skin is strongly promoted by sleep and this vasodilation may be a way for the body to facilitate the distribution of endogenous defense agents,” the researchers write. “With a lack of sleep, blood flow to the skin is reduced and, according to the raters, faces look more pale after not sleeping.”

But looking a bit rough after a bad night of sleep isn’t an entirely bad thing, the researchers point out. The appearance that comes with sleep deprivation could be an adaptive one that helps people be “left alone in order to recover from their current state.” Keeping away is good for the well-rested as well — while there’s inherent bias against people who look sick, alert people get to stay away from the aggressive, moody behavior of those who need a few more hours in bed.

Photos via Flickr, Giphy