Laurence Theil lounges around in bed, with attendants to massage her back and bring her breakfast on a tray. But it isn't exactly a life of leisure: The French nurse has spent 50 straight days confined to bed for space research.

She is not allowed to stand or sit up, ever; a 24-hour surveillance camera makes sure of that. She showers lying down and even jogs in bed, strapped into a vertical treadmill that makes her feel she's running up a wall.

"Running while you're on your back takes some getting used to," says 36-year-old Theil, one of 24 Europeans who volunteered to boldly go where few women have gone before — to bed for 60 days.

The joint study by the European Space Agency, the French space agency CNES, the Canadian Space Agency and NASA will fill in unknowns about protecting women astronauts from the side effects of weightlessness.

Twelve women completed the study six months ago; the rest are now in bed at a hilltop research center overlooking the red tile roofs of Toulouse, in southwest France.

Day and night, in comfy T-shirts and pajamas, they lounge on hospital beds tilted so their feet are 6 degrees higher than their heads — provoking a physical reaction akin to weightlessness.

In space, as in the off-kilter beds, fluids shift to the upper body, which can make the face swell, disrupt digestion and bring on vertigo.

In microgravity, the magic of floating makes up for it.

"Human beings become halfway between a fish and a bird," said Roberto Vittori, a European Space Agency astronaut who visited the clinic to cheer volunteers on.

But it also means the lower body starts wasting away: At first people lose up to 3 percent of muscle mass a week in parts of their legs, and bone mass in segments of their lower bodies drops about 2 percent in a month, said Dr. Peter Jost, project manager for the study.

Bed-rest studies are frequent in space study, and testing nutritional supplements and exercise routines is important to keep astronauts fit for longer missions.

A manned trip to Mars, for example, would expose astronauts to reduced or zero gravity for up to three years. When they land on the red planet, they have to be strong enough to work.

Few of the studies have focused on women, however, because most astronauts are male, Jost said. Today, however, one in five NASA astronauts is female.

"We know there are subtle differences, but we don't know which, and to what extent," Jost said.

One question is how much exercise women need to best avoid loss of bone mass, given that strenuous workouts can modify women's complex hormonal and menstrual systems.

In the first group of bed-rest volunteers, all lost bone mass at first, but after six months they are back to normal, said Dr. Arnaud Beck, the coordinator in charge of medical ethics.

One surprise find: Past research from short-term missions suggested that women astronauts are more susceptible to dizziness after returning home. But that did not prove true during tests in Toulouse. One possibility is that the difference may fade or disappear during long-term missions.

Volunteers from across Europe are paid $17,500 each to give constant blood samples, submit to electrocardiograms and take tests for bone density and muscle strength. Their urine — all of it — is stored and analyzed. The women receive counseling and will have checkups for three years.

Anne Bouchet, a 36-year-old translator, is used to putting her body to the test hiking in the French Alps. She signed up to test the opposite extreme — sloth.

"I thought it would be interesting to live confined for a long period, without moving at all," said Bouchet, talking to The Associated Press over the phone because face-to-face interviews are prohibited.

Theil feels in good shape and says she has even lost some flab on her legs. On Sunday she will test them on solid ground.

She might be wobbly, if the first group is anything to go by. The most stir-crazy wanted to run, but were permitted only to walk — unsteadily.