Beauty sleep is no myth. A new study out of Stockholm University published in the Royal Society Open Science journal finds a lack of sleep makes a person look “significantly” less attractive. And that’s not all. Swollen eyelids and dark circles around the eyes might also prevent people from hanging out with you, say researchers, who photographed 25 volunteers without makeup after two consecutive nights of about 7.5 hours of sleep and again after two consecutive nights of slightly less than 4.5 hours of sleep, reports the Los Angeles Times. The scientists then showed the photos to 122 strangers, who were asked to rate the subject’s attractiveness, health, sleepiness, and trustworthiness, then decide if they’d be likely to socialize with that person. A person’s trustworthiness was rated about the same no matter which photo participants were viewing. But participants were generally less likely to socialize with subjects suffering from a lack of sleep, whom they considered less attractive, less healthy, and more sleepy than subjects who’d gotten a good night’s sleep, according to the study. Researchers have a clear theory on this: They suggest humans inherently avoid those who look tired or ill as a means of steering clear of disease. A tired person might also be viewed as a risk—perhaps they’ll slip up in their drowsiness—whereas people who look more energetic are probably viewed as being better company, researchers say.