As their muscles diminish - one participant has lost almost four kg - medical staff at the study clinic MEDES in Toulouse, France monitor them closely.
Two groups of volunteers follow a short but intense exercise routine on a vibrating plate that exercises leg muscles as they absorb the up-and-down motion. Straps pull them onto the plates with a force equivalent to 100-200 kg while the pillownauts perform upside-down leg-presses for a few minutes.
The exercise routine is repeated every three days. "It is not a lot of time but you definitely feel it," says Eddy, from the exercise group.
One group is doing the exercise and taking protein supplements just as bodybuilders eat to increase their muscles. The third, control, group is being monitored to compare results.
The pillownauts have access to the Internet and all agree that social contact is keeping them motivated.
Rooms are shared by two volunteers and procedures are discussed between all volunteers in group chats. The experiments are run by doctors from France, Germany and Italy, Austria and Canada.
The study is part of ELIPS, the European Programme for Life and Physical Sciences and co-funded by the French national space agency, CNES.
Eddy adds, "The research results will be used in everyday life and that makes me proud."
The volunteers are looking forward to the end of the year, when they can leave their sloping beds after 21 days for four months of rehabilitation. They will return for another two 21-day stints, each time taking part in another of the three groups.