Space can be an unfriendly place for the human body, with microgravity conditions and other factors tampering with our physiology, and of main concern is brain physiology. Astronauts who traveled to the International Space Station or on NASA space shuttles on missions lasting at least six months experienced significant expansion of part of their brains (the cerebral ventricles).
Researchers found that it takes three years for the ventricles to fully recover after such space journeys. Suggesting that an interval of at least three years would be advisable between longer space missions.
The induced Age-related ventricular enlargement, caused not by microgravity but by brain atrophy, can be associated with cognitive decline.
Learn More here: https://www.nature.com/articles/s41598-023-33331-8