So, the scientists next compared 13 men and women who habitually slept 6.5 hours or less per night at home with 14 men and women who regularly slept about 8 hours. On the weekends, the short sleepers slept extra hours, indicating that their weekday patterns resulted from social constraints rather than biological constitution, Van Cauter notes.
Researchers verified the test volunteers' sleep patterns at home for a week and then brought them into the lab for a glucose-tolerance test. The short sleepers showed 50% more insulin resistance than the others did.
Short sleep may accelerate the onset of diabetes, Van Cauter speculates.
"If you are predisposed to diabetes, and you might become diabetic at 55, are you becoming diabetic at 45?" she asks.
In further experiments, sleep-deprived test volunteers showed other hormonal changes that promote weight gain. Men who were held to 4 hours a night had markedly reduced 24-hour leptin concentrations compared with when they were fully rested, Van Canter's research team reported at the 2001 Association of Professional Sleep Societies meeting in Chicago. Leptin is a hormone that signals satiety and regulates energy balance; mice that lack leptin overeat and become morbidly obese.
Van Cauter reported that although the men's food intake was adequate, the dip in leptin they exhibited was equivalent to that seen in people under-fed by 1,000 calories a day, for three days. In other words, the leptin signal was telling the men's bodies that they were short nearly a pound's worth of calories. That misleading signal might cue the body to slow metabolism, increase fat deposition, and overstimulate appetite.
A separate, ongoing study is examining sleep restriction and hunger. When held to 4 hours of sleep, volunteers reported being hungrier than when they had adequate slumber, Van Cauter says. The sleep-deprived people overwhelmingly asked for candy, starchy foods, and salty snacks such as potato chips.
"There were no cravings for fruits and vegetables," she quips.
Animal studies also suggest that partial sleep deprivation leads to hormonal and metabolic changes. In one experiment, rats that normally sleep 8 to 10 hours a day were restricted to 4 hours of daily sleep for a week, mirroring Van Cauter's studies in people.
During that time, the rats showed increased concentrations of stress hormones and an altered hormonal response to stressful situations, such as being confined in a small space.
Initially, such changes may help the body cope with lack of sleep, says Peter Meerlo of the University of Groningen in the Netherlands, who reported the results in the May Journal of Neuroendocrinology. If stress hormones are chronically altered, however, sleep deprivation may have adverse health effects, Meerlo speculates.
Modest sleep deprivation may also be associated with low-grade inflammation, which can lead to a host of cardiovascular problems, according to Alexandros N. Vgontzas of the Pennsylvania State University College of Medicine in Hershey.
Trying to mimic the modest chronic sleep loss that many people in the United States endure, Vgontzas and colleagues deprived 25 healthy young men and women of just 2 hours of sleep per night for a week. The scientists measured blood concentrations of immune-system molecules called cytokines, which are normally secreted during inflammation and infection.
After a week of sleeping 6 hours per night, the test volunteers had higher blood concentrations of the cytokine IL-6, than they did in their pre-deprivation state. Furthermore, the men, but not the women, had increased concentrations of the cytokine TNF-a.
Increased cytokines may reflect pervasive inflammatory action, the researchers speculated last June in San Francisco at the annual meeting of the Endocrine Society.