U. COLORADO (US) — The metabolic cost of missing one night’s sleep is the equivalent of walking slightly less than two miles.
“We found that people do expend more energy when they are awake in bed than when they are asleep,” says Kenneth Wright, associate professor of integrative physiology at the University of Colorado-Boulder, who says eight hours of sleep saves roughly 135 calories over eight hours of wakefulness.
“While the amount of energy savings for humans during sleep may seem relatively small, it actually was a little more than we expected,” Wright says.
Details of the study, published in the Journal of Physiology, show that compared to a typical night of sleep, the amount of energy expended by study subjects during 24 hours of sleep deprivation was up about 7 percent.
In contrast, energy expenditure decreased by about 5 percent during the recovery episode, which included 16 hours of wakefulness following the sleep deprivation night, then eight hours of recovery sleep.
“Understanding the function of sleep, especially in humans, is considered one of the most important scientific enigmas,” Wright says.
The study, which included seven young adult subjects, was tightly controlled. All participants were required to stay in bed for the entire three-day study. Their diets met individual daily energy requirements, and the content and timing of each meal was exactly at the same time each day during the lab study. The subjects spent the sleep deprivation night in bed watching movies, reading, and talking.
The first day of the study consisted of a typical 16 hours of wakefulness followed by eight hours of sleep. Days two and three included 40 hours of total sleep deprivation followed by eight hours of recovery sleep.
As part of the study, researchers studied the effects of sleep stages ranging from light sleep to rapid-eye movement sleep to deep, “slow wave” sleep and awakenings from sleep on whole body energy expenditure.
The study indicated the most energy was expended during natural arousals from sleep, which occurred less often during the eight-hour sleep episodes following sleep deprivation.
The amount of energy saved during sleep by the study subjects likely would have been higher if they were allowed to continue sleeping after the eight hours of recovery sleep rather than being awakened, which was the final step in the study.
The study may have implications for those with sleep disorders such as insomnia or sleep apnea. Insomnia, marked by difficulty going to and staying asleep, and sleep apnea, marked by frequent arousals from sleep, may mean such people “are burning the furnace at a higher rate at night because their sleep is disturbed,” says Wright.
It’s likely that the metabolic costs of sleep deprivation would have been higher if the subjects had not been restricted to bed rest and had opportunities to walk around and perform various tasks.
Other studies have shown that sleep deprivation reduces the levels of leptin—a hormone responsible for telling the brain that the body is satiated—which could mean late-night snacking by “free-ranging” humans.
“One question we have is why humans don’t conserve more energy during sleep,” he says. “We think there are multiple functions of sleep, and that some energy conserved during sleep may be redistributed to support other important physiological processes.”
Some energy conserved by sleep might be used for nighttime physiological activities like immune-system function, the strengthening of connections between neurons in the brain as a result of daily learning, and experience, and hormone synthesis and release.
One of the health areas scientists are very interested in is how sleep loss may contribute to weight gain and obesity.
Wright stresses that energy expenditure during sleep deprivation is neither a safe or effective strategy for weight loss, and that other studies have shown chronic sleep deprivation is associated with impaired cognition.
More research is needed to understand how short nighttime sleep schedules, typically six hours or less a night across many days, contribute to weight gain and obesity.
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