Spending at least six months in microgravity can cause astronauts’ brain volumes to increase, causing pressure to build up in their heads and creating vision problems
Source:
https://www.newscientist.com/article/2240405-long-space-flights-can-increase-the-volume-of-astronauts-brains/?utm_campaign=RSS%7CNSNS&utm_source=NSNS&utm_medium=RSS&utm_content=home