The astronauts returning Wednesday after five months on the International Space Station may miss sleeping in space. According to new research published in the August issue of the American Journal of Respiratory and Critical Care Medicine, astronauts sleep more soundly in space than at home on Earth.

Scientists from the University of San Diego, Brigham and Womens Hospital, Harvard Medical School and the NASA Ames Research Center studied five healthy astronauts before, during and after space flight to investigate the effects of weightlessness on sleep-disordered breathing. The microgravity of space, they found, resulted in a 55 percent reduction in the apnea-hypopnea index (AHI)a measure of the incidence of temporarily not breathing (apnea) and very shallow, slow breathing (hypopnea). What's more, snoring was essentially eliminated while the subjects were in space, dropping from 17 percent of total sleep time to less than 1 percent. (That's good news, given the confined living quarters of most spacecraft.)

The small study helps quantify the effects of gravity on the respiratory system and related sleep disorders, which are important because gravity acts differently on a person lying down than on someone standing up, the researchers say. "This is the first direct demonstration that gravity plays a dominant role in the generation of apneas, hypopneas and snoring in healthy subjects," the authors conclude.