People who consistently get too little sleep face bigger concerns than daytime fatigue and crankiness. Over the long term, sleep deprivation also increases the risk of serious health problems including obesity and type 2 diabetes.
Scientists have come up with a number of plausible explanations for this increased risk. Various studies have shown, for instance, that how much we sleep can affect blood sugar levels, hormones that control appetite, and even the brain's perception of high-calorie foods.
A small new study, published today in the Annals of Internal Medicine, adds a key piece to the puzzle by drilling down to the cellular level: Sleep deprivation, the study found, impairs the ability of fat cells to respond to insulin, a hormone that regulates metabolism and is involved in diabetes.
In the study, seven healthy young men and women spent a total of eight days and nights in a sleep lab. They were allowed to sleep normally on four of the nights, and on the other nights they were limited to just 4.5 hours. In order to neutralize the effects of appetite or overeating, the researchers strictly controlled the participants' meals and calorie intake.
After the four nights of sleep deprivation, blood tests revealed that the participants' overall insulin sensitivity was 16% lower, on average, than after the nights of normal sleep. Moreover, their fat cells' sensitivity to insulin dropped by 30%, to levels typically seen in people who are obese or who have diabetes.
"This is the equivalent of metabolically aging someone 10 to 20 years just from four nights of partial sleep restriction," says Matthew Brady, Ph.D., the senior author of the study and an associate professor of medicine at the University of Chicago. "Fat cells need sleep, and when they don't get enough sleep, they become metabolically groggy."
Specifically, the participants' fat cells—which were collected via biopsy and analyzed—required nearly three times as much insulin to activate an enzyme known as Akt, which plays a crucial role in regulating blood sugar. If insulin resistance of this sort becomes persistent, excess sugar and cholesterol can accumulate in the blood, increasing the risk of diabetes and heart disease.
Previous sleep-lab studies have found that insufficient sleep can affect overall insulin sensitivity, but this is the first to identify a concrete cellular mechanism that might underlie the well-established links between sleep, diabetes, and obesity.
"This takes the research on the effect of sleep deprivation on metabolism one step further, by revealing a molecular mechanism involved in the reduction of total body insulin sensitivity," says Nathaniel F. Watson, M.D., co-director of the University of Washington Medicine Sleep Center, in Seattle, who was not involved in the study.
"If you want to make a causal argument that short sleep is causing diabetes," Watson adds, "one of the key elements is coming up with a physiological mechanism by which this would happen."
Brady and his coauthors aren't yet sure how exactly fat cells recognize and register sleep deprivation. One possibility, they say, is that lack of sleep triggers the body's stress response, leading to the release of the stress hormones cortisol and norepinephrine, which are associated with insulin resistance.
The new findings will need to be confirmed in different populations and settings. The study included only seven people (and just one woman), and they were all young, healthy, and lean, so the results can't necessarily be extrapolated to people who are older or overweight.
Likewise, the sleep deprivation in the study was relatively drastic and short-lived. It's unclear whether less severe sleep deprivation over longer periods of time—a more common real-world scenario—would have the same effect on fat cells.
If the results are borne out in the future, the good news is that the treatment for the type of insulin resistance seen in the study is straightforward: sleep more.
Sleep is "as important to your health as a healthy diet and exercise," Watson says. "Until somebody invents a procedure or a pill that's going to approximate all aspects of sleep, really what you're left with is what is a pretty simple treatment… Just turn off the computer and go to bed earlier."