Cosmic radiation, stress, insomnia and trying to re-orient your body's balance system mean the astronaut's brain is constantly adopting while in space.

Like our muscles, bones aren't needed much in microgravity. Without exercise, they deteriorate and become weak and brittle.

In space, muscles aren't really needed to hold up the body. Over time, they become weaker and deteriorate (worse). Astronauts must exercise everyday to prevent this from happening.

Blood is normally pulled towards our feet by gravity. But in space, the blood and fluid is pulled towards the head giving astronauts puffy faces.

In space, hearts don't have to work as hard to pump blood around the body. So eventually, they may shrink, which can cause problems for astronauts when they return to earth.

Many astronauts suffer from some vision loss. Although it is not completely understood why. It is believed to be due to increased pressure inside the head. Cosmic rays, usually absorbed by our atmosphere, also creates momentary flashes of light.