Beauty sleep is a real thing, scientists studying the "negative effects of restricted sleep on facial appearance and social appeal" have found.
In the study, published in Royal Society Open Science, researchers discovered that a few nights of bad sleep can make you appear "significantly" less attractive to others, as well as "less healthy".
In even more bad news for poor sleepers, they found that people are less inclined to want to socialize with you if you're tired.
The Karolinska Institute in Stockholm sent 25 university students home with a kit to measure night-time movements and check they were getting the amount of sleep they reported they were. The students were asked to get a good night's sleep for two nights in a row. After that, they were photographed by the researchers.
A week later, they were asked to restrict themselves to a measly four hour's sleep for two nights in a row. After this, they were photographed again.
The researchers then took to the streets of Stockholm to show the two sets of photographs to 122 strangers, asking them a range of questions from "how much would you like to socialize with this person in the picture?" to "how trustworthy is this person?"
The team, lead by Dr Tina Sundelin, found that strangers were good at assessing how tired a person was, and that they rated the sleepy set of photographs as less attractive than the photos of the same people when they were well-rested.
They also found that those rating "were less inclined to socialize with individuals who had gotten insufficient sleep," even rating them as less healthy than those who had slept properly.
Whilst there was no difference in perceived trustworthiness, the study suggested that "naturalistic sleep loss can be detected in a face and that people are less inclined to interact with a sleep-deprived individual".
In short, people can tell when you're tired and think you are less attractive when you haven't got enough sleep.
The authors said that their findings made sense in evolutionary terms. "An unhealthy-looking face, whether due to sleep deprivation or otherwise, might activate disease-avoiding mechanisms in others," they wrote. Basically, when you see someone who looks sick, the last thing you want to do is hang out with them.
The paper also noted that "although attractiveness may only be weakly related to actual health, faces that look healthy are also considered attractive."
"This study is a good reminder of how important sleep is to us," Dr Gayle Brewer, a psychologist at the University of Liverpool told BBC News. However, Dr Sundelin also pointed out, "I don't want to worry people or make them lose sleep over these findings though. Most people can cope just fine if they miss out on a bit of sleep now and again."