|This article does not cite any references or sources. (December 2006)|
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 9.8 m/s2. If the body is in free fall (for example, during skydiving) with no other momenta (rotation, etc.) there is no falling sensation. This almost never occurs in real-life falling situations because when the faller leaves his support there are usually very significant quantities of residual momenta such as rotation and these momenta continue as the person falls, causing a sensation of dysphoria. The faller doesn't fall straight down but spins, flips, etc. due to these residual momenta and also due to the asymmetric forces of air resistance on his asymmetric body. While velocity continues to increase, the downward acceleration due to gravity remains constant and is not sensed. Increasing drag force may even cause a feeling of ascent.
The vestibular apparatus also detects spatial orientation with respect to visual input. 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. 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.
The vestibular system works with other sensorimotor systems in the body, such as visual system (eyes) and skeletal system (bones and joints), to check and maintain the position of our body at rest or in motion. The vestibular apparatus functions by detecting forces that act upon bodies including gravity. There are two sections in the labyrinth that are helpful for accomplishing those tasks: the semicircular canals and the otolithic organs.
A balance disorder is a condition that makes a person feel nausea, disorientation or dizziness, as if moving, spinning or falling even though steady. Balance disorder can be caused by medication, problems of inner ear or the brain.
Hypnic jerk, also called as hypnagogic jerk, is a normal reaction that can be caused by anxiety, caffeine, a dream, or discomfort of sleeping. Basically, hypnic jerk is the feeling when people experience a sudden muscle twitch and feel like they are falling while sleeping and sometimes dreaming. About 70% of people have experienced hypnic jerk.
There is no completely accurate explanation of this phenomenon, but most experts[who?] agree that as muscles begin to slack right after one falls asleep, the brain senses these relaxation signals and misinterpret as one is falling down. Then the brain will send signals to muscles to keep the balance.
Studies[which?] also showed that there is a correlation between the frequency of hypnic jerks and anxiety. Fatigue, discomfort of the bed, highly intense exercise and stress might cause people to jerk during sleeping.