That's right: beauty sleep is real. This was monitored with a kit that recorded their night-time movements. People were rated by strangers as less healthy and approachable when they had exhausted faces.
The team, lead by Dr Tina Sundelin, found that strangers were good at assessing how exhausted 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.
In a scientific study published by Royal Society Open Science journal and funded by Karolinska Institutet and Stockholm University, researchers photographed 25 participants after days of both restricted and normal sleep.
In short, people can tell when you're exhausted and think you are less attractive when you haven't got enough sleep. They explain that blood flow to the skin is strongly promoted by sleep.
However, while the study found that people were found to be less attractive when lacking sleep, it discovered no change in other's perception of how trustworthy a person appears.
A person's attractiveness score generally suffered when they were looking exhausted.
After each recorded night, researchers took pictures of the students and showed them to another group of people who were asked to analyze the photos and rate each student based on attractiveness, health, sleepiness, and trustworthiness.
According to the study, "Having an unhealthy-looking face, whether due to sleep deprivation or otherwise, might thus activate disease-avoidance mechanisms in others and render one's surroundings less socially inclined". The Karolinska Institute researchers says this makes sense in evolutionary terms. In other words, people don't want to hang around with people who might be ill, whereas someone who looks energetic and fit will hold lots of appeal. "Most people can cope just fine if they miss out on a bit of sleep now and again".
You know that it's important to get a good night's sleep, and odds are you probably try to do the best that you can. The last thing we want when we're running on four hours of snoozing is a chatty Cathy strolling up to us and trying to make friends.