A new study says the habit of sleeping in on weekends may not be such a good idea for your waistline -- or your health.
sleep in: 睡懒觉;睡过头
"Weekend catch-up sleep is not protective," said Dr. Vsevolod Polotsky, director of sleep research at the Johns Hopkins University School of Medicine. "The bottom line of this study is that even if you sleep longer on weekends, if you continue to sleep poorly, you will still eat too much, and you will still gain weight."
"Sleeping in on the weekends doesn't correct the body's inability to regulate blood sugar if that weekend is followed by a workweek or school week full of insufficient sleep," said study author Kenneth Wright Jr., who directs the sleep lab at the University of Colorado in Boulder.
"And when we go back to getting too little sleep again," Wright said, "we're doing things that could be negative for our health long-term."
The American Academy of Sleep Medicine recommends at least seven hours of sleep each night for adults and much more for children.
The study, published in the journal Current Biology, assigned 36 healthy young men and women to three groups that had different sleep requirements over a total of 10 days. None of the participants had newborns in the home or any health impairments that would affect the quality of their sleep.
The first group had the opportunity to sleep for nine hours each night for the 10 days. The second group was restricted to only five hours of sleep a night for the same duration, while the third was restricted to five hours Monday through Friday but allowed to sleep as long as they wanted on the weekend and go to bed as early as they liked on Sunday night. Come Monday, that third group was put back on the deprived sleep schedule of only five hours a night.
Both of the sleep-deprived groups snacked more after dinner and gained weight during the study, men much more than women. The sleep-deprived men showed a 2.8% increase in their weight, while women's body size went up by only 1.1%; men who slept in on the weekend showed a 3% increase in weight, while women's body size went up 0.05%.
Gaining weight while sleep-deprived isn't surprising, Wright said.
"One of the things we and others have found in the past is that when people don't sleep enough, they tend to eat more, partly because their body is burning more calories. But what happens is that people eat more than they need and therefore gain weight."
That could be in part, Polotsky said, because hunger hormones are affected by a chronic lack of sleep.
"The hormone leptin decreases appetite, while the hormone ghrelin increases appetite," explained Polotsky, who was not involved in the study. "We know from previous research that sleep deprivation causes leptin to drop and ghrelin to rise, so you're hungry."
What was surprising is what happened to the group who slept in on weekends.
"Even though people slept as much as they could, it was insufficient," Wright said. "As soon as they went back to the short sleep schedules on Monday, their ability of their body to regulate blood sugar was impaired."
Not only that, but the weekend recovery group showed increased sensitivity to insulin in both their muscles and their livers, a result not found in the second group on restricted sleep. That's important, Wright said, because the muscle and liver are two of the most important tissues that take up blood sugar after eating.
"That helps us understand why is it that when we don't get enough sleep, we have an increased risk for things like diabetes," he added, because "short, insufficient sleep schedules will lead to an inability to regulate blood sugar and increases the risk of metabolic disease in the long term."
Metabolic syndrome is an array of symptoms such as fat around the waist, abnormal cholesterol, high blood sugar and high blood pressure, all of which can raise the risk of heart disease, stroke and diabetes.
One of the reasons the weekend group may have been more affected is because their circadian rhythm, or biological clock, had been altered, depriving the body of certain hormones.
"If you catch up during weekends, you habitually eat later, because the circadian clock is shifting," Polotsky said. "Add in after-dinner snacks; the sleep-deprived eat much more after dinner, as well."
So what's a sleep-deprived person to do if a catch-up over the weekend isn't a good option? Polotsky sums it up in two words: "Sleep longer."