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The ill effects of getting too little sleep—weight gain, increased diabetes risk and more—have been known for a long time, but now researchers think they’ve identified the cellular process for how sleep deprivation takes its toll. Just a few nights of scanty sleep reduces the response of fat cells to the hormone insulin, which puts the body, at least temporarily, into a state akin to pre-diabetes.

A small clinical trial published Monday in the Annals of Internal Medicine compared fat cells in seven college students both before and after they slept 4.5 hours a night for four consecutive nights and found that their cells were 30 percent less responsive to insulin after their period of sleep deprivation.

“It’s the equivalent of metabolically aging these students by as much as 20 years,” said study co-author Matthew Brady, a molecular medicine researcher at the University of Chicago. “This is a real-world scenario where people cram for work deadlines during the work and then try to catch up on sleep on the weekends.”

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What’s not known is how quickly the fat cells return to their regular metabolic state after a period of sleep deprivation, Brady added.

When cells become resistant to insulin, the body resorts to releasing more of the hormone in order to sop up excess sugar that’s circulating in the blood to use as energy or to store in fat cells. An increase in insulin often leads to an increase in appetite, excess calorie consumption, and weight gain.

The study certainly has limitations: It was small and its results need to be verified through larger studies. Also, it’s not known whether fat cells become significantly less sensitive to insulin if you skimp a little on sleep—getting six hours a night instead of seven or eight—for a longer period of time. “It is possible that acute and chronic sleep loss may exert different effects on metabolism,” wrote British researchers in an editorial that accompanied the study.

But Brady said a growing body of evidence points to sleep deprivation being a contributing factor to obesity and obesity-related diseases like diabetes. “Eating well, exercise, and getting enough sleep are all probably equally important.”