Many people dream of traveling into outer space. However, spending too much time in space may cause health problems, according to NASA. That's why astronauts shouldn't spend more than six months on the International Space Station (ISS) at a time.
Muscle (肌肉) and bone loss
In a weightless environment, astronauts don't need to use their muscles to support themselves. Therefore, their muscles start shrinking (萎缩). To keep their muscles strong, astronauts need to exercise about 2 hours each day on the ISS.
The weightless environment also causes astronauts' bones to lose minerals, with density(密度) dropping at over 1 percent per month. The rate of bone loss for elderly people on Ear this 1 to 1.5 percent per year.
Sleep disorder(混乱)
ln space, the sun rises every 90 minutes. It is hard for astronauts to get used to this light-dark cycle when they sleep. Also, they need to sleep in a sleeping bag that is fixed to a wall, which is uncomfortable.
Astronauts now have individual sleeping pods (分离仓) that help shut out some of the light. Still, studies have found that most astronauts only sleep about six hours each night, even though they should have eight and a half hours.
Brain changes
Scientists have found that the position of the brain changes during spaceflight. It shifts upward within the skull (头骨). This can affect how the brain deals with how we see, stay balanced and move around. That's why some astronauts can't control their movement and balance after returning to Earth. Scientists have also found that astronauts may get used to this change after going on more spaceflights. But they still need to do more research.
