People often joke about needing their "beauty sleep," but researchers in Sweden have provided scientific evidence that we really do.
People kept awake for 31 hours developed droopy, swollen eyelids, bloodshot eyes and dark circles under their eyes, according to the researchers, who relied on objective assessments provided by 40 observers who compared photographs of 10 people before and after sleep deprivation.
"We confirmed that sleep-deprived people are perceived as more fatigued, less attractive, sadder and less healthy than when they are rested, confirming the colloquial notion of beauty sleep," the researchers wrote in their paper, published in the journal Sleep.
The sleep-deprived also appeared to have more facial wrinkles, a consequence of poor sleep unverified by science until now - at least in humans.
Rats deprived of sleep quickly develop nasty skin lesions on their paws and tails.
"It is well-known that when you sleep the blood flow to your skin increases dramatically," says Tina Sundelin of the Karolinska Institute in Stockholm, who led the research team.
"It thus seemed likely that sleep loss would affect the skin."
The 10 sleep-deprived subjects said they felt mentally fatigued, and they developed a droopy mouth that the observers thought indicated sadness.
"Sleep loss is indeed related to a negative mood," Sundelin said.
"We also seem to be more emotional in general when we don't get enough sleep."
The research provides strong support for the folk wisdom that good sleep improves facial appearance.
In fact, a 2010 paper in the British Medical Journal co-authored by Sundelin actually carried the title Beauty Sleep: experimental study on the perceived health and attractiveness of sleep-deprived people.
That study, like the recent study in the journal Sleep, also concluded that sleep-deprived people look less attractive and less healthy than they do when well-rested.
This has implications for interpersonal interactions, according to the researchers.
"Since faces contain a lot of information on which humans base their interactions with each other, how fatigued a person appears may affect how others behave toward them," Sundelin said in a news release announcing the results of the research.
The findings also suggest the appearance of fatigue is something people are willing to pay handsomely to avoid.
"The desire to look less fatigued is one of the primary motivators for undergoing cosmetic surgery," the authors of the Sleep study point out.
But it's not all about vanity, according to Sundelin. Sleep may be involved in health as well, which she and her colleagues hope to demonstrate next.
"We are actually conducting studies at the moment looking at disease and appearance," she said.
"There are studies out there on how stress, disease and physical or mental exertion affect health, but a lack of studies specifically looking at how facial cues of fatigue relate to health."