When the body is at rest, gravity takes over from inertia. The otoliths fall to the bottom of the appropriate sacs and lie on and bend the hairs at the base. Slight motions sway the hairs at the end of the canals to which fluid has flowed. The most common problem of balance is motion illness, which includes seasickness . Motion illness is caused by the shaking of the balance labyrinth or by contradictions between the visual balance system and the labyrinth system.
Astronauts have the same problems as sailors, but for a different reason.
In the free-fall conditions of space, gravitational force is almost nonexistent, but inertia is unchanged. Some diseases directly affect the semicircular canals, causing all the effects of motion illness even when movement is normal, as in changing position. Chief among these disorders is Meniere's disease. Infections of the ear or of the mastoid region of the skull can also invade the semicircular canals and affect balance. Blood pressure in the head needs to be maintained as the body changes position.