Sleep is a mystery. Although no one knows exactly why, it’s required for good health. But now, scientists have found a surprisingly clear connection between sleep and a healthy body: the regulation of sugar levels in the blood.

The new studies, all online December 7 in Nature Genetics, describe the first genetic link between sleep and type 2 diabetes, a disease marked by high blood sugar levels.

In the United States, the number of people with type 2 diabetes is increasing, according to a 2006 paper in the journal Circulation; while the average amount people sleep is dwindling, according to a sleep survey by the Centers for Disease Control and Prevention.

The investigations by three international teams of researchers suggest the trends of rising diabetes and falling sleep are linked via a protein that senses the sleep-inducing hormone melatonin. The research places bodily rhythms, including the clock that sets human sleep cycles, squarely in the blood sugar business.