Researchers claim to have found the first proof that getting a regular eight hours a night really does make you appear healthier and more attractive.
When untrained observers were shown photographs of the faces of volunteers who had been deprived of sleep, they judged them to be less healthy and less attractive than photographs of the same volunteers when well-rested.
The finding will be welcomed by parents trying to get image-conscious teenagers off to bed, but is a blow to the cosmetics industry marketing beauty by the (expensive) bottle.
"If you want to look healthy and attractive, it is way better to get a good night's sleep and a lot cheaper than other beauty treatments," said John Axelsson, of the department of clinical neuroscience at the Karolinska Institute in Stockholm, who led the study.
The differences between the tired and fresh faces were slight, but observers subconsciously picked up visual clues, he said. "They are small signals, but if you spend a few seconds looking at a face, you instinctively read them. These are judgements you make about people without thinking."
The ability to read signs of sleep deprivation could have an evolutionary advantage, since an attractive face is a sign of good health. This triggers sexual attraction, the choice of a mate and the successful transmission of genes. A good night's sleep before a date could mean the difference between a lifetime's happiness and a taxi home alone.
Dr Axelsson said sleep deprivation caused physiological changes which could be seen in the face. The immune system was affected, increasing the risk of infections, glucose regulation was weakened and blood pressure was raised. "We have seen the effects in shift workers – at age 35 they shows signs of developing atherosclerosis [heart disease]," he said.
But isn't it obvious that someone who appears tired would be perceived as less attractive? Not so. "People who look tired may work more and earn more money. I assumed tiredness was related to healthiness, but I did not think it would be related to attractiveness." The study is published in the British Medical Journal's Christmas issue, which draws together the year's most off-beat research.
Another paper assesses the Danish myth that submerging your feet in alcohol will get you drunk. It isn't true.
For the study, a dozen men and a dozen women slept for no more than five hours on the first night, which was spent at home, followed by a second night in a sleep laboratory, when they were not allowed to sleep at all. Photos were taken in the afternoon, after 31 hours of continuous wakefulness, and compared with photos taken after a full night's sleep of eight hours.
The photos were judged by 65 observers, who rated them for attractiveness and whether they looked tired or healthy. A few of the volunteers were perceived as healthier, less tired and more attractive in the sleep-deprived condition. "It remains to be evaluated in follow-up research whether this is due to random error or associated with specific characteristics of observers or the sleep-deprived people they judge," the authors say.
Dr Axelsson said further studies were under way into which facial traits were important in making the judgements. Attempts were also being made to replicate the findings.