https://www.theregister.co.uk/2019/04/12/nasa_twin_study/

Spending your days floating in space may give you a nice break from Earth, but be warned, your body might not fully recover when you decide to come back down.

NASA astronaut Scott Kelly found this out the hard way when he circled the International Space Station (ISS) for 340 days. Kelly is among the group of 559 people who have gone up to space so far but he is a particularly useful human guinea pig to study what effects space has on the human body for one reason: He’s a twin.