Although space sickness gets better within a few days, it causes headaches, lethargy and sleepiness. After some time, the face becomes pale, nausea sets in and, in really bad cases, some may vomit severely. Space sickness can suddenly strike, causing some astronauts to vomit without any prior warning. While in the 1-G environment of Earth, the eyes, the inner ear and other body parts, such as muscles, bones or skin, provide the body with the information it needs to cope adequately and maintain itself in the area it is in. But in a weightless environment, the information that reaches the ears, muscles and skin is not stable and the only organ that functions properly in terms of receiving information is the eyes.
Astronauts lose their sense of knowing exactly where they are.
One of these effects occurs when fluids in the body rise up when entering a weightless environment. On Earth, gravitational pull ensures bodily fluids always travel in a downward direction. But in a weightless environment, this power is lost and fluids rise up into the head. After a few days in space the blood that had shifted toward the upper body adjusts itself to the same levels as on Earth. Human bodies adjust themselves according to the environment they find themselves in and in space the body automatically reduces the amount of blood it contains.