Tallying the caloric cost of an all-nighter

The body conserves energy during sleep, more so after missing zzz's

Staying up all night clearly taxes the body, but scientists have only now added up the exact bill. By measuring the actual number of calories the body expends to fuel an all-nighter versus a good night’s sleep, researchers from the University of Colorado at Boulder calculate that a full night of sleep helps the body conserve as much energy as is in a glass of warm milk.

Missing a night of sleep forces the body to burn about an extra 161 calories than it would have during eight hours of sleep (not counting what’s used in moving around while awake), but it’s no weight-loss miracle: The body tries to make up for the deficit by saving more energy than usual the next day and night, researchers report in the January Journal of Physiology.

The measurements, the first to put precise numbers on how much total energy people use in a 24-hour period while asleep, awake or recovering from a night of sleep deprivation, help bolster a theory that an important function of sleep is to save energy (SN: 10/24/09, p. 16).

To measure how much energy people use during sleep in a more rigorous way than has been done before, Kenneth Wright, a physiologist at the University of Colorado, and his colleagues studied seven people. Each of the healthy young volunteers lived inside a sealed room for three days. The volunteers were on bed rest the entire time and ate the same amount of calories at the same time each day. The researchers continually monitored the subject’s brain waves and how much oxygen and carbon dioxide the person breathed in and out. From there, the team could calculate each person’s energy use during each stage of sleep and waking.

“This is a Herculean effort,” neurobiologist Paul Shaw of Washington University in St. Louis says of the study. “This will be the gold standard going forward.”

Recently, scientists had dismissed energy conservation as sleep’s most important mission. “Sure, there’s energy savings, but it’s not worth much. It’s a hot dog bun or a cup of milk’s worth,” Shaw says of many scientists’ attitude toward the amount of calories the body saves during sleep. But “small differences can have large consequences,” says Shaw, who was not involved in the current research.

Wright notes that eating just 50 extra calories per day over an extended period can lead to obesity. So, he contends, the amounts of energy savings associated with sleep aren’t trivial. But would-be dieters shouldn’t interpret the new data as pointing to sleep deprivation as a weight-loss plan, Wright says. Many studies have linked chronic sleep deprivation with obesity and other health problems (SN: 10/24/09, p. 28).

After staying up all night, volunteers burned about 28 fewer calories during eight hours of recovery sleep than they had during a full night of regular sleep. And the energy conservation didn’t stop there. In the 24-hour period during which people caught up on missing sleep, they burned about 228 fewer calories than during a comparable period in which they were sleep-deprived. Overall, when people slept normally, they expended 96 more calories than they did on days when they were making up for lost sleep.

Volunteers were limited to eight hours of recovery sleep, and the effect might have been even greater if the researchers had allowed volunteers to fully sleep off their sleep debt, Wright says.

Shutting down muscles and other body functions during sleep might save the body even more energy than measured, but the body does have crucial energy requirements in other areas. The body may be using energy to fuel essential sleep functions such as rewiring connections between brain cells, boosting the immune system and regulating hormones, the researchers speculate. Sleep-deprived people are diverting some of the energy needed for those important processes, says Wright. “It’s not worth the cost.”

Chronic sleep deprivation caused by conditions like sleep apnea and insomnia might exact a high energetic toll, Wright says. Each time volunteers awakened from sleep at night, energy consumption shot up, even if the episode lasted only seconds, the researchers report. People with sleep apnea rouse many times during the night, raising the possibility that they are tapping energy reserves needed for sleep’s other functions, Wright says.

Next, the researchers hope to learn whether missing a few hours of sleep each night over several days has different energy costs than staying up all night.

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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