Attention, to all dieters: You can cut all the calories you want to lose weight – but without enough sleep, you won’t be losing the right kind.

According to a study published online Monday in the Annals of Internal Medicine, cutting your time in bed from 8.5 hours to 5.5 hours causes you to lose proportionally less fat. Ten overweight dieters who cut their caloric intake by 10% lost a comparable amount of weight – about 3 kilograms, or 6.6 pounds – but the type of weight they lost was very different, depending on how long they slept.

For dieters who had a full night’s worth, more than half of the weight they lost was fat. But when the researchers cut three hours off their bedtime, only a quarter of the weight the study participants lost was fat. That means the other 75% being burned was nonfat mass – such as protein, valuable building blocks of muscle and other body tissues.

How could this be? The researchers theorize that it’s because of the way sleep levels affect the levels of ghrelin, a hormone that stimulates hunger and promotes fat retention – two symptoms you don’t want when you’re trying to lose weight. Sleep loss, the authors write, “amplifies” these ghrelin-associated changes.

“Thus,” they write, “the increased loss of fat-free body mass during the short-sleep condition of our study may be due to increased conversion of body protein into glucose to support the more prolonged metabolic needs of the waking brain and other glucose-dependent tissues.”

Because all the dieters stuck to the same caloric regimen during the study – keeping the sleep-deprived dieters from giving into those hunger pangs – the experiment may have even downplayed the potential negative effects of sleeplessness.

Moral of the story? Put aside the computer, or that late-night TV show, and get some sleep. Compared to exercise and diet, it’s the easiest part of healthy weight loss.