Americans sleep less than they used to, and this could be part of the reason why more of us are now overweight, says David Dinges, chief of the Division of Sleep and Chronobiology at the University of Pennsylvania School of Medicine. Over the past 40 years, Americans have cut their snooze time by one to two hours a night. We now sleep less than people in any other industrialized country. And researchers are discovering that sleep affects hormones that regulate satiety, hunger and how efficiently you burn calories. Too little sleep may make you hungry, especially for calorie-dense foods, and may prime your body to try to hold on to the calories you eat. It may also boost your insulin levels, which increases the risk of heart disease and diabetes.
The Sleep-Weight Link
“Obesity is obviously a very complex issue, and no one is suggesting that lack of sleep is the cause of the obesity epidemic,” says Carl Hunt, director of the National Center on Sleep Disorders Research at the National Institutes of Health in Bethesda, Md. “But new research certainly supports the idea that sleeping less may be a previously unknown but important contributor to the obesity epidemic in the U.S.”
The link between sleep and weight was first noticed in the 1990s, when European researchers were puzzling over why so many children were getting heavier. “They were surprised to discover that it wasn’t how much TV a child watched but how much sleep the child got that best predicted whether he or she was overweight,” says Dinges. “The less children slept, the heavier they were.”
Researchers in the United States are finding the same link in adults. In the ongoing Wisconsin Sleep Cohort Study, which tracks the sleep habits of nearly 3,000 middle-aged state government employees, those who reported that they typically slept less than eight hours a night were more likely to be overweight. And researchers at Columbia University in New York City found that people who slept six hours a night were 23 percent more likely to be obese than people who slept between seven and nine hours. Those who slept five hours were 50 percent more likely—while those who slept four hours or less were 73 percent more likely—to be obese.
Why would people who sleep less weigh more? James Gangwisch, a psychiatric epidemiologist at Columbia, says it may have “more to do with what happens to your body when you deprive it of sleep, as opposed to the amount of physical activity you get.” What happens involves two hormones: Leptin, which is released by fat cells, signals the brain to stop eating. Ghrelin, which is made in the stomach, is a signal to keep eating. The two influence whether you go for a second helping or push yourself away from the table. “Studies have shown that leptin levels are lower and ghrelin levels are higher in people who sleep fewer hours,” says Gangwisch.
In the Wisconsin Sleep Cohort Study, those who slept for five hours had 15 percent lower leptin levels and 15 percent higher ghrelin levels than those who slept for eight hours. While the study wasn’t designed to prove whether sleep deprivation causes changes in leptin and ghrelin levels, new research at the University of Chicago suggests that it does. When Eve Van Cauter and co-workers limited 12 healthy young men to just four hours of sleep for two consecutive nights, their leptin levels were 18 percent lower and their ghrelin levels were 28 percent higher than after two nights of sleeping for 10 hours.
“The combination of low leptin and high ghrelin is likely to increase appetite,” says Wisconsin Sleep Cohort Study researcher Emmanuel Mignot of Stanford University. In fact, the men in Van Cauter’s study said that they were more hungry—and that they’d be more likely to eat salty foods like chips and nuts; sweets like cake, candy, and ice cream; and starchy foods like bread, cereal, and potatoes—after four hours of sleep than after 10 hours. Compounding the problem: The brain interprets a drop in leptin as a sign of starvation. So it responds not only by boosting hunger, but by burning fewer calories. That means you put on more weight even if you don’t eat any more food.
Sleep deprivation “also affects insulin resistance and blood glucose levels, which are two important components of the metabolic syndrome,” says Hunt of the National Center on Sleep Disorders Research. The metabolic syndrome, also called insulin resistance syndrome, is a cluster of symptoms that increases the risk of heart attack, stroke, and diabetes. Signs of the syndrome are abdominal obesity, low HDL (“good”) cholesterol, and elevated (though not necessarily high) triglycerides, blood pressure and blood sugar.
When the University of Chicago’s Van Cauter and her colleagues limited 11 healthy men in their twenties to four hours of sleep for six straight nights, “it brought them to a nearly prediabetic state.” Their bodies were 40 percent less able to clear glucose from their blood and 30 percent slower in releasing insulin than when they were allowed to sleep for 12 hours. In fact, four hours of sleep for six consecutive nights gave the young men the insulin sensitivity of 70- or 80-year-olds.
“Sleep really affects everything. We are not wired biologically for sleep deprivation. We’re the only animal that intentionally sleeps less than we need to.” says Van Cauter.