It depends if we feel this sensation during the transition from wakefulness to sleep or during REM sleep.
In the first case, the mechanism is physiological. When our body ceases to exercise ”active” control (handled by the cortical areas of the brain) on movement our body is crossed by a series of muscular jerks —sleep starts or hypnic jerks. If the transition occurs too early (i.e. when our conscious part has still not gone to sleep) these jerks may be perceived as a feeling of emptiness, and our “baffled” mind associates it with our perception of emptiness: falling.
Why this happens is not clear (according to some, it’s an evolutionary heritage from when we were sleeping on trees and muscles relaxation could mean a fall). Anyway, according to the American Academy of Sleep Medicine, if the feeling happens too often and disturbs our rest, we must reduce: caffeine, stress, anxiety or hard physical activity in the evening.
The sensation of falling that we feel during REM sleep is not caused by a physiological mechanism and, in this case, for an explanation we should disturb dreams and psychology.
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