Researchers in Sweden found people rated photographs of strangers as less attractive and healthy when the people in the photographs had less sleep.
The study used photographs of healthy, mainly young, students taken after either two nights of normal sleep (around eight hours a night) or two nights of restricted sleep (around four hours a night).
The photos were rated by 122 strangers, who were asked how much they would like to socialise with the people in the photographs, and how healthy, attractive, trustworthy and sleepy they looked.
The study found that on average, people were 2.1% less likely to want to socialize with people who'd had less sleep.
It's unclear how significant this finding is in real life, or what effect it might have on people not getting enough sleep. If you're having difficulty sleeping, whether or not other people want to socialize with you may be the least of your worries.
Persistently poor sleep can increase the chances of obesity and diabetes, and worsen conditions like depression and anxiety.
The study was carried out by researchers from Karolinska Institute and Stockholm University in Sweden and was funded by the two institutions and published in the journal Royal Society Open Science.
This was a psychological study, using volunteers. This type of study can show the effects of experimental conditions on volunteers, but doesn't necessarily tell us what happens to people with sleep problems in real life.
Researchers recruited 14 female and 11 male students, mostly in their early 20s but ranging from 18-47 years old. All 25 students had their photograph taken twice – once after two nights of sleep restriction and once after two nights of normal sleep.
The photos were viewed by 122 members of the general public from Stockholm, 65 of them women, who gave ratings on a number of questions.
The researchers looked at the results to see if there was a difference between people's ratings of photos taken when people were sleep restricted, or when they'd had normal sleep.
For the photographs after normal sleep, people were told to go to bed for around eight hours, between 10pm and midnight until between 6am and 8am.
Before the sleep deprivation photographs, people were told to go to bed for around four hours, between midnight and 2am until between 4am and 6am.
They used actigraphs (special monitors) to measure activity so the researchers could check the students had followed the instructions properly.
The average difference in hours of sleep between the normal and restricted sleep was 3.5 hours a night, adding up to seven hours less sleep than normal over two nights.
All photographs were taken at the same time of day, by the same photographer, with people wearing no make-up and hair scraped back from the face.
Raters were asked to look at 50 photos (two from each person) and say on a scale of one to seven:
The researchers excluded ratings from people whose ratings showed low variability (less than 0.5 standard deviation between scores on normal sleep and restricted sleep photos) because they say this could indicate "low motivation to adhere to the instructions of the task".
People's average ratings were mostly in the middle of the seven-point scale on all questions, with averages between three and five for people who'd had normal sleep.
Raters' scores suggested they were less willing to socialize with people who'd been sleep restricted, but only by 0.15 points on a seven-point scale (around 2.1%).
Compared with average ratings after normal sleep, average ratings on a seven-point scale were:
There was no difference in the trustworthiness scores between normal sleep and sleep deprivation. Analysis showed only about a third of people's reduced willingness to socialize with sleep-restricted people was explained by the findings on attractiveness, health and sleepiness. In other words, something other than attractiveness, health or sleepiness was putting people off.
The researchers say their study "indicates that restricted sleep affects facial appearance negatively and decreases others' willingness to socialize with the sleep deprived person". They say it confirms previous findings that people completely deprived of sleep for one or two nights are judged to look less healthy and attractive, and extends the findings to "less substantial and more natural" sleep loss conditions.
Most people who have looked in the mirror after a sleepless night won't be surprised to hear that a poor night's sleep makes you look less attractive and healthy. It may not be particularly welcome news that your appearance could also put people off talking to you. But the study results show only a very small impact of sleep deprivation on people's perceptions of appearance.
While the results are statistically significant, it's hard to know how you would notice a 2% drop in a stranger's willingness to socialise with you and studies like this, which include only a limited demographic (in this case Swedish students aged around 22, mostly white) may have little relevance to anyone who doesn't fit that profile.
More important are the known health effects of sleep problems. An occasional late night is very different from persistent difficulties in getting to sleep or staying asleep.
Regular poor sleep can raise your risk of diabetes, heart disease and obesity, and is linked to mental health problems like anxiety and depression.
There are plenty of things you can try yourself to increase your chances of getting a good night's sleep. But if you've tried these and you're still struggling to sleep, talk to your GP.
Updated May 18 2017