The key to looking attractive and healthy is as simple as a good night's sleep, new research suggests.
Experts say they have uncovered the first scientific backing for the concept of "beauty sleep".
Such an expression reveals that "an individual's sleep history may play an integral part in the perception and judgments of his or her attractiveness and health", they said.
"To date, the concept of beauty sleep has lacked scientific support, but the biological importance of sleep may have favoured a sensitivity to perceive sleep-related cues in others.
"It seems warranted to explore such sensitivity, as sleep disorders and disturbed sleep are increasingly common in today's 24-hour society and often co-exist with some of the most common health problems, such as hypertension and inflammatory conditions."
The team, from the Karolinska Institute in Sweden, recruited 23 healthy adults aged 18 to 31.
Smokers were excluded from the research and no alcohol was allowed for two days before the experiment.
The participants were photographed after eight hours of sleep, and then again after a period of sleep deprivation – 31 hours of being continuously awake after a short night's sleep.
The photographs were standardised, meaning the people were the same distance from the camera, wore no make-up and wore the same expression.
The pictures were then presented in a random order to 65 untrained observers, who ranked the participants on a scale according to how healthy, attractive or sleep-deprived they looked.
The results showed that those who were sleep-deprived were rated as less healthy – typically scoring 63 versus 68 after a full night's sleep.
They were also seen as more tired and less attractive.
Writing in the British Medical Journal online, the team concluded: "Our findings show that sleep-deprived people appear less healthy, less attractive, and more tired compared with when they are well rested."