Chronic sleep loss can reduce the capacity of even young adults to perform basic metabolic functions such as processing and storing carbohydrates or regulating hormone secretion, report researchers from the University of Chicago Medical Center in the October 23 issue of The Lancet. Cutting back from the standard eight down to four hours of sleep each night produced striking changes in glucose tolerance and endocrine function -- changes that resembled the effects of advanced age or the early stages of diabetes -- after less than one week.
Although many studies have examined the short-term effects of acute, total sleep deprivation on the brain, this is the first to investigate the impact of chronic, partial sleep loss on the body by evaluating the metabolism and hormone secretion of subjects subjected to sleep restriction and after sleep recovery.
"We found that the metabolic and endocrine changes resulting from a significant sleep debt mimic many of the hallmarks of aging," said Eve Van Cauter, Ph.D., professor of medicine at the University of Chicago and director of the study. "We suspect that chronic sleep loss may not only hasten the onset but could also increase the severity of age-related ailments such as diabetes, hypertension, obesity and memory loss."
Cutting back on sleep is an extremely common response to the time pressures of modern industrial societies. The average night's sleep decreased from about nine hours in 1910 to about 7.5 hours in 1975, a trend that continues. Millions of shift workers average less than five hours per work day. Previous studies, however, have measured only the cognitive consequences of sleep loss.
Van Cauter and colleagues Karine Spiegel and Rachel Leproult chose to focus instead on the physiologic effects of sleep loss, how sleep deprivation altered basic bodily functions such regulating blood-sugar levels, storing away energy from food and the production of various hormones.
They followed 11 healthy young men for 16 consecutive nights. The first three nights the subjects were allowed to sleep for eight hours, from 11 p.m. to 7 a.m. The next six nights they slept four hours, from 1 a.m. to 5 a.m. The following seven nights they spent 12 hours in bed, from 9 a.m. to 9 p.m. All subjects received identical diets.
The researchers constantly assessed each volunteer's wakefulness and heart rate. They performed sleep studies on the last two eight-hour nights, the last two four-hour nights, and the first and last two 12-hour nights. They performed glucose tolerance tests on the fifth day of sleep deprivation and the fifth day of sleep recovery and monitored glucose and hormone levels every 30 minutes on the sixth day of deprivation and of recovery.
They found profound alterations of glucose metabolism, in some situations resembling patients with type-2 diabetes, during sleep deprivation. When tested during the height of their sleep debt, subjects took 40 percent longer than normal to regulate their blood sugar levels following a high-carbohydrate meal. Their ability to secrete insulin and to respond to insulin both decreased by about 30 percent. A similar decrease in acute insulin response is an early marker of diabetes.
The differences were particularly marked when tested in the mornings. "Under sleep debt conditions, our young lean subjects would have responded to a morning glucose tolerance test in a manner consistent with current diagnostic criteria for impaired glucose tolerance," note the authors. Impaired glucose tolerance is an early symptom of diabetes.
Sleep deprivation also altered the production and action of other hormones, dampening the secretion of thyroid stimulating hormone and increasing blood levels of cortisol, especially during the afternoon and evening. Elevated evening cortisol levels are typical of much older subjects and are thought to be related to age-related health problems such as insulin resistance and memory impairment.
All of these abnormalities quickly returned to baseline during the recovery period, when subjects spent 12 hours in bed. In fact, as the subjects spent more than eight hours a night in bed, their laboratory values moved beyond the "normal" or baseline standards, suggesting that even eight hours of sleep does not produce the fully rested state. Young adults may function best after more than eight hours of rest each night.
"While the primary function of sleep may very well be cerebral restoration," note the authors, "our findings indicate that sleep loss also has consequences for peripheral function that, if maintained chronically, could have long term adverse health effects."
Funding for this study was supplied by the Research Network on Mind-Body Interactions of the MacArthur Foundation (Chicago), the U.S. Air Force Office of Scientific Research and the National Institutes of Health.
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