Unremitting low-grade inflammation can damage the inner walls of the arteries, which sometimes leads to vessel narrowing, high blood pressure, stroke, and heart disease. Also, cytokines have been associated with insulin resistance, diabetes, and obesity.
Cytokines cause fatigue. By overproducing cytokines, a person's body is probably trying to say, "Go to sleep," Vgontzas says. Test participants fell asleep faster and slept more deeply when they were sleep deprived, demonstrating that their bodies were trying to compensate for the reduced sleep time, he adds.
However, the more efficient sleep didn't thwart the cytokine response, which lasted the entire week. The volunteers were also sleepier and performed more poorly on an alertness test at the week's end than at the beginning of the experiment.
"There are some researchers, even in the sleep area, that say that these extra couple of hours of sleep are not important," Vgontzas concludes. "Our data say that 6 hours is not good for healthy, young people."
In a separate study, sleep of 4 hours a night for 10 nights was also associated with increased concentrations of C-reactive protein, another key inflammation mediator. Boosts in C-reactive protein might have a negative effect on health, says David F. Dinges of the University of Pennsylvania School of Medicine in Philadelphia, a researcher on the study.
Currently, the evidence linking increases in inflammatory molecules and cardiovascular disease is stronger for C-reactive protein than for the cytokines, adds Dinges' collaborator Janet Mullington of Beth Israel Deaconess Medical Center in Boston.
A recent epidemiological study also showed a direct association between short sleep and heart disease. After controlling for other factors, researchers found that men who slept 5 hours or less a night had twice as many heart attacks as men who slept 8 hours, report Japanese scientists in the July Occupational and Environmental Medicine.
Obstructive sleep apnea, a condition marked by temporary pauses in breathing during sleep, is a natural model of chronic sleep loss because people with the condition repeatedly wake up through the night. Several recent studies have linked sleep apnea to elevated blood concentrations of IL-6, TNF-a, and C-reactive protein, and to high blood pressure, cardiovascular problems, and stroke.
However, it's hard to tease out whether these effects result from lack of oxygen due to the apnea, loss of sleep, or both, says sleep-apnea researcher Virend K. Somers of the Mayo Clinic in Rochester, Minnesota.
Virtue or Indulgence?
Some scientists remain skeptical that sleeping 8 hours should be the next great health virtue. They say that getting from the current evidence to a firm link between sleep loss and disease requires a giant leap of faith.
"Some people don't have time to sleep, or they'd rather watch television. Should we condemn them before the evidence is in?" asks Daniel F. Kripke of the University of California, San Diego. Kripke argues that telling people that they'll get sick if they don't sleep enough may, ironically, worry them into insomnia.
Kripke and his colleagues published results in February that, on the surface, contradict the idea that more sleep is good for you. During a large epidemiological study lasting 6 years, people were more likely to die if they initially reported sleeping 7.5 hours or more a night than if they reported sleeping 5.5 to 7.5 hours a night. The researchers took into account such factors as age, weight, diagnosed illness, and medication use.
The amount that the average person in the United States sleeps per night, about 7 hours, is consistent with good health, Kripke concludes. "People who are saying you should sleep more don't have the evidence," he adds.
Anything more than 7 hours is optional sleep, which can be taken for relaxation and indulgence but is not necessary for good health, agrees sleep scientist Jim A. Horne of Loughborough University in England.
Pennsylvania State's Vgontzas disagrees. Underlying depression and sickness probably explain the apparent association between sleeping 8 or more hours and increased mortality in Kripke's study, he says.
However, neither Horne nor Kripke is swayed by the studies that show physiological changes in sleep-deprived subjects in the lab. The changes are probably real, they agree, but may not be meaningful either because they're not severe enough to cause long-term health effects or because they're artifacts of the experimental situation.
The immune system probably does crank up and go on red alert when a person is awake longer, Home explains. After all, a body is more likely to come across pathogens when it's up and about. But there's no evidence that this activity undermines health, he says.
The argument that people are evolutionarily programmed to sleep 8 or 9 hours a night doesn't hold up, at least in European history, Home adds. Hundreds of years ago, people worked 14- or 15-hour days and were lucky to get 6 hours of sleep at night, he says.
Moreover, their sleep patterns were different from those of modem slumberers. In England, people went to bed an hour after sundown, got up a few hours later for a midnight meal, and then slept a few more hours until sunup, Horne says. People are probably designed to sleep 6 or 7 hours at night and take a short afternoon nap, he suggests.
Bleary Picture. Research on sleep deprivation and health is still in its infancy, Van Cauter admits. Nevertheless, she maintains that 8 or more hours of sleep a night is optimal.
"To suggest to people that you can maintain average sleep time of 6 hours and get on the road and drive your truck is criminal," she asserts. It's too early to say whether sleep loss causes disease, but it certainly hinders performance and diminishes safety, she says.
There's no reason to believe that sleeping more than 8 hours could be harmful or that insomnia could be good for you, Vgontzas adds.
"My general appraisal of the literature is, in terms of more or less normal variations in sleep amount and effects on health, we really don't know," reflects sleep specialist Alan Rechtschaffen of the University of Chicago.
There's a consensus that the extreme-below 6 hours a night-isn't advisable, he says. Beyond that, distinguishing the health effects, if any, of 6 vs. 7 vs. 8 hours of sleep is going to take large, well-controlled studies that follow people over long periods. Optimal sleep also probably varies with age and gender, Vgontzas says.
So, as to whether millions of cases of obesity, diabetes, and cardiovascular disease could be prevented if people in the United States were simply to increase their sleep from 7 to 8 hours a night, the issue hasn't been put to bed. But the early data are at least provocative.
"We have all the dots or a lot of the dots, and it looks like there's a picture there, but the science that actually connects those dots hasn't been done yet," Dinges says.
Sleep disorders, such as sleep apnea, that were once seen as mere nuisances are now recognized by the community of sleep researchers as serious health concerns. The data suggest that even young, healthy people should give more consideration to sleep.
Of all the health prescriptions out there, it may be easier to convince people to change their sleep habits than to make other lifestyle changes.
After all, in a time when we are constantly told to eat more broccoli, eat less chocolate, and do more push-ups, wouldn't it be nice if hitting the snooze button were just what the doctor ordered?