From The Art and Popular Culture Encyclopedia
A sensation of falling occurs when the labyrinth or vestibular apparatus, a system of fluid-filled passages in the inner ear, detects motion. This sensation usually occurs when the person involved is falling. The same system also detects rotary motion. A similar sensation of falling can be induced when the eyes detect rapid apparent motion with respect to the environment. This system enables us to keep our balance by signalling when a physical correction is necessary.
When a human is in free fall in an orbiting spacecraft, or in an aircraft in a steep dive, the sensation of falling is constant, and the sensation of there being an "up" and a "down" is missing or much attenuated. Some medical conditions, known as balance disorders, also induce the sensation of falling. In the early stages of sleep, a falling sensation may be perceived in connection with a hypnic jerk, sometimes awaking the sleeper abruptly.