The idea of people needing "beauty sleep" has acquired some scientific backing, according to a Swedish study.
People deprived of sleep for long periods appear less attractive and more unhealthy than those who are well rested, say researchers.
Volunteers were photographed after eight hours sleep and again after being kept awake for 31 hours.
Observers scored the sleep-deprived participants as less healthy and less attractive, the BMJ reports.
The concept of beauty sleep is well known.
But, according to researchers at the Karolinska Institute in Stockholm, it has lacked scientific support.
The team asked untrained observers to rate the faces of 23 young men and women who had been photographed after a normal night's sleep and then after a night of sleep deprivation.
The photographs were standardised so that people were the same distance from the camera, wore no make-up and used the same expression.
The authors wrote in their paper published in the British Medical Journal: "Sleep deprived people are perceived as less attractive, less healthy and more tired compared with when they are well rested."
They say the results may be useful in a medical setting, helping doctors to pick up signs of ill-health in their patients.
Commenting on the study, Derk-Jan Dijk, Professor of Sleep and Physiology at the Surrey Sleep Research Centre, said the effects of sleep loss on beauty may be even more dramatic than the photographs show.
He said: "The photographs were taken during the daytime when the biological clock promotes wakefulness.
"Can you imagine how sleep loss makes you look at night or early in the morning when the circadian clock (body clock) promotes sleep?"