Nasa is paying volunteers to spend two months in bed.
The test subjects are spending 60 days and nights lying down at a special centre in Cologne as part of a study, undertaken jointly with the European Space Agency (ESA), into the effects of weightlessness.
Although similar studies have been carried out before, this one - which is looking specifically at the possible benefits of artificial gravity to astronauts - is the first to be conducted jointly by the two agencies.
According to the ESA, volunteers are required to be "healthy people between the ages of 20 and 45.
"Most importantly, they should be highly motivated," the agency says.
Jennifer Ngo-Anh, ESA team leader for research, said: "This study allows us to address the issue of muscular atrophy caused by weightlessness, but also other stressors such as cosmic radiation, isolation and spatial restrictions."
Researchers use constant bed rest as a means of simulating the muscle and bone wastage which is caused by the weightlessness experienced in space.
Astronauts on the International Space Station currently do around two-and-a-half hours of exercise each day to combat the problem.
But researchers believe that artificial gravity could help to mitigate the effects.
Although they are spending their days and nights in bed - and receiving a reported £14,000 for doing so - the study may not be the most restful experience for the 24 participants.
The 12 women and 12 men will constantly lie at an angle of six degrees, meaning their head will be below horizontal, and they must have a shoulder touching the bed at all times.
Researchers will then test the effects of various techniques on their bodies.
Some will experience artificial gravity in the form of a special centrifuge at the German Aerospace Centre, which will spin them around and encourage blood to flow back towards their feet.
Nasa warns that while spending all day in bed "might sound great", the studies can be arduous and "boredom sets in quickly".
Family and friends are allowed to visit, but the agency warns that: "Daily routine - showering, getting dressed, eating, exercising - takes a lot of time when you cannot stand up to do them."
Nasa encourages participants to "set a goal such as learning a new language or taking a class online" to help pass the time productively.
The study began on March 26.