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The best path to a healthy weight may be a good night’s sleep.

For years researchers have known that adults who sleep less than five or six hours a night are at higher risk of being overweight. Among children, sleeping less than 10 hours a night is associated with weight gain.

Now a fascinating new study suggests that the link may be even more insidious than previously thought. Losing just a few hours of sleep a few nights in a row can lead to almost immediate weight gain.

Sleep researchers from the University of Colorado recruited 16 healthy men and women for a two-week experiment tracking sleep, metabolism and eating habits. Nothing was left to chance: the subjects stayed in a special room that allowed researchers to track their metabolism by measuring the amount of oxygen they used and carbon dioxide they produced. Every bite of food was recorded, and strict sleep schedules were imposed.

The goal was to determine how inadequate sleep over just one week — similar to what might occur when students cram for exams or when office workers stay up late to meet a looming deadline — affects a person’s weight, behavior and physiology.

During the first week of the study, half the people were allowed to sleep nine hours a night while the other half stayed up until about midnight and then could sleep up to five hours. Everyone was given unlimited access to food. In the second week, the nine-hour sleepers were then restricted to five hours of sleep a night, while the sleep-deprived participants were allowed an extra four hours.

Notably, the researchers found that staying up late and getting just five hours of sleep increased a person’s metabolism. Sleep-deprived participants actually burned an extra 111 calories a day, according to the findings published last week in The Proceedings of the National Academy of Sciences.

But even though we burn more calories when we stay awake, losing sleep is not a good way to lose weight. The light sleepers ended up eating far more than those who got nine hours of sleep, and by the end of the first week the sleep-deprived subjects had gained an average of about two pounds.

During the second week, members of the group that had originally slept nine hours also gained weight when they were restricted to just five hours. And the other group began to lose some (but not all) of the weight gained in that first sleep-deprived week.

Kenneth Wrightdirector of the university’s sleep and chronobiology laboratory, said part of the change was behavioral. Staying up late and skimping on sleep led to not only more eating, but a shift in the type of foods a person consumed.

“We found that when people weren’t getting enough sleep they overate carbohydrates,” he said. “They ate more food, and when they ate food also changed. They ate a smaller breakfast and they ate a lot more after dinner.”

In fact, sleep-deprived eaters ended up eating more calories during after-dinner snacking than in any other meal during the day. Over all, people consumed 6 percent more calories when they got too little sleep. Once they started sleeping more, they began eating more healthfully, consuming fewer carbohydrates and fats. Dr. Wright noted that the effect of sleep deprivation on weight would likely be similar in the real world although it might not be as pronounced as in the controlled environment. The researchers found that insufficient sleep changed the timing of a person’s internal clock, and that in turn appeared to influence the changes in eating habits. “They were awake three hours before their internal nighttime had ended,” Dr. Wright said. “Being awakened during their biological night is probably why they ate smaller breakfasts.”

The effect was similar to the jet lag that occurs when a person travels from California to New York.

Last fall, The Annals of Internal Medicine reported on a study by University of Chicago researchers, who found that lack of sleep alters the biology of fat cells. In the small study — just seven healthy volunteers — the researchers tracked the changes that occurred when subjects moved from 8.5 hours of sleep to just 4.5 hours. After four nights of less sleep, their fat cells were less sensitive to insulin, a metabolic change associated with both diabetes and obesity.

“Metabolically, lack of sleep aged fat cells about 20 years,” said Matthew Brady, an associate professor of medicine at the University of Chicago and the senior author on the study.

“These subjects were in their low 20s but it’s as if they were now middle-aged in terms of their response. We were surprised how profound the effects were.”

Both Drs. Wright and Brady noted that because their studies lasted only days, it was not clear how long-term sleep deprivation affects weight, and whether the body adjusts to less sleep.

Dr. Brady said that while better sleep would not solve the obesity problem, paying more attention to sleep habits could help individuals better manage their weight.

In the future he hopes to study whether a focus on better sleep could improve the health of people in middle age who are overweight or prediabetic.

“Telling someone they need to sleep more as a way to improve their metabolic health, we think would be more palatable,” said Dr. Brady. “We think sleep is very underappreciated.”