A sensation of falling occurs when the labyrinth or vestibular apparatus, a system of fluid-filled passages in the inner ear, detects changes in acceleration.
This sensation can occur when a person begins to fall, which in terms of mechanics amounts to a sudden acceleration increase from zero to roughly 9.81 m/s2.
A similar sensation of falling can be induced when the eyes detect rapid apparent motion with respect to the environment.
In the early stages of sleep, a falling sensation may be perceived in connection with a hypnic jerk, sometimes awaking the sleeper abruptly.
A balance disorder is a condition that makes a person feel nausea, disorientation or dizziness as if moving, spinning or falling even though steady.