A study, led by Eve Van Cauter, Ph.D., researcher at the University of Chicago Medical School in Chicago, IL, examined the effects of varied amounts of sleep on 11 men ages 18-to-27. The men spent eight hours in bed per night (fully rested period) for the first three nights, four hours per night (sleep deprivation period) for the next six nights and 12 hours per night for the last seven nights (recovery period).
Results showed that after being deprived of sleep, the men's bodies metabolized glucose less efficiently. Levels of the stress hormone cortisol were also higher during sleep deprivation periods than when the study subjects were fully rested. Elevated cortisol levels have been linked to development of memory impairments and age-related insulin resistance and may impair athletic recovery. The study was published in the October 23 issue of The Lancet. "The alterations in metabolic and endocrine function were striking," Dr. Van Cauter said. "After only one week of sleep restriction, young, healthy males had glucose levels that were no longer normal. That's a rapid deterioration of the body's functions."
The study showed that the rate at which the body was able to clear glucose from the blood after an insulin injection was nearly 40 percent slower during periods of sleep deprivation than in the sleep-recovery condition. Glucose effectiveness - defined as the ability of glucose to mediate its own disposal independently of insulin - was 30 percent lower in the sleep-deprived subjects.
Dr. Van Cauter said the effects of sleep deprivation on glucose metabolism were similar to those found in the elderly. She therefore concluded that chronic sleep deprivation may have long-term harmful effects on the body. However, Dr. Van Cauter said the study also showed that the negative effects of sleep deprivation could be corrected entirely by normal sleep, as demonstrated in this study. "Getting a normal amount of sleep could be a health-promoting behavior," she said. "Just as a lack of sleep can harm the body, getting sleep can help it."
Most of what we know about sleep deprivation has to do with immune function and brain function. This study is interesting because it shows that sleep deprivation can negatively impact physiology that is critical for athletic performance - glucose metabolism and cortisol status.
Glucose and its storage form, glycogen, are the main sources of energy for any athlete, whether it be a 50-meter freestyle swimmer or 100-mile ultramarathoner. Storage of glucose in muscle and liver is particularly important for the endurance athlete. In a sleep-deprived state glycogen storage may be slowed, preventing the athlete from topping his or her fuel tanks. In endurance events exceeding 90 minutes this can lead to hypoglycemia and the dreaded bonk.
Because cortisol is a catabolic (breakdown) hormone, elevated levels - as seen in the sleep-deprived subjects - may interfere with tissue repair and growth. Over longer periods this could prevent an athlete from adapting properly to heavy training and predispose that athlete to overtraining and injury.
"The findings are interesting, and I'd like to see more," added Michael Bonnet, Ph.D., director of the sleep laboratory at the Dayton Veterans Administration Medical Center in Dayton, Ohio.
"But we need longer-term studies to see if the effects of sleep loss are temporary, and if they are still reversible over those extended periods."
Dr. Van Cauter agreed that more research is necessary before any definitive statements can be made regarding the effects of sleep deprivation on the body. "But these results indicate that a chronic lack of sleep may affect metabolic function as much as living a sedentary lifestyle," she said. "Sleep has almost become a sin these days, and that thinking really needs to change."
For the endurance athlete looking for every edge, it appears that proper sleep during heavy training and during lead-up periods before key competitions is critical for optimal performance.
Spiegel, Leproult and Van Cauter, Impact of sleep debt on metabolic and endocrine function. The Lancet (1999;354:1435-1439).