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You may think you can get by on four or five hours of sleep, but your fat cells beg to differ.

Lack of shut-eye reduces fat cells' ability to respond to insulin, a hormone that regulates energy, researchers have found.

In a study published in the Annals of Internal Medicine, participants were limited to 4-and-a-half hours in bed each night. After four nights of reduced sleep, their fat cells behaved like those of obese people and patients with Type 2 diabetes.

"Fat cells need sleep," says study author Matthew Brady, associate professor of medicine and vice-chair of the committee on molecular metabolism and nutrition at the University of Chicago.

Previous research has linked sleep deprivation to weight gain and obesity. However, this is the first study to investigate sleep's role in energy metabolism at the cellular level.

Brady and colleagues recruited six men and one woman for the study. All were young, lean and healthy. In the first study period, they spent 8.5 hours a night in bed for four consecutive nights. Four weeks later, they spent 4.5 hours in bed for four nights. Their food intake was strictly controlled in both study periods.

On the morning after each four-night stint, participants took an intravenous glucose tolerance test, which measures the body's insulin sensitivity. The researchers also removed abdominal fat cells near each participant's navel to measure how the fat cells responded to insulin.

After four nights of inadequate rest, all seven participants showed a decline in insulin response. Total-body insulin response decreased by an average of 16 per cent, while the insulin sensitivity of their fat cells decreased by 30 per cent.

"We were surprised by the magnitude of the change," Brady says.

When fat cells become insulin resistant, it's a double whammy for a person trying to lose weight. Metabolism is likely to drop while the appetite goes into overdrive, Brady says, adding that decreased insulin sensitivity could lead to metabolic disorders such as Type 2 diabetes.

Brady and colleagues didn't study how long it would take for the participants' fat cells to return to normal functioning, but he predicts that after four nights of poor sleep, "you'd bounce back within several days."

The bigger question, he says, is "can we take sick people, such as those with the common combination of sleep apnea, obesity and diabetes, improve their sleep and make them better?"

Although research is under way, increasing the duration and quality of a patient's sleep is not a proven intervention for obesity and related disorders, notes Ari Shechter a sleep expert at the New York Obesity Research Center. But he adds, "I think anything that would improve sleep continuity, sleep duration and improve sleep architecture would have the potential to improve the symptoms associated with metabolic disorders, including obesity and glucose regulation."

The obesity epidemic in North America coincides with dramatic changes in our sleep habits, Shechter points out. He and other researchers estimate the average duration of sleep per night has decreased by about two hours since the 1960s. He calls the University of Chicago study "a step in the right direction, finding this cellular mechanism that is underlying the associations we see on a more global level."

But he adds the study should be replicated with a larger sample size and include people with metabolic disorders to see if their fat cells react in similar ways to inadequate sleep.

Despite the lack of evidence that improving sleep helps weight loss, obesity experts are urging individuals to get more Zs.

The Canadian Obesity Network recently listed sleep and stress management as the top intervention in its obesity management guidelines – ahead of diet or exercise.

Studies have shown that people who don't get enough sleep consume extra calories, often in the form of unhealthy snacks, says Arya Sharma, an obesity specialist at the University of Alberta and scientific director of the Canadian Obesity Network.

Sharma notes the quality of a person's sleep is as important as the duration. "There are people who need less sleep than others, but I think one of the best measures is whether people are sleepy."

He adds that chronic fatigue is a barrier to weight loss. "If you're not getting good sleep, that's going to affect your activity levels, your energy levels and your eating behaviour," he says.