There's an explanation for the sensation of falling as you drift off to sleep.

The sensation of falling just as one drifts off to sleep has a scientific explanation rooted in the body's complex transition into sleep. This phenomenon, known as a hypnic jerk, occurs when a person experiences a sudden muscle contraction, often accompanied by a feeling of plummeting or missing steps on non-existent stairs. These episodes typically happen during the brief period between wakefulness and sleep, and while they can startle individuals awake, they are generally harmless and common among humans. As the body shifts from being fully alert to entering the state of sleep, there is a gradual handover of control between different brain systems. The networks in the brainstem and hypothalamus maintain wakefulness, while other systems promote sleep and suppress these wake-promoting networks. During this transition, which takes several minutes, the signals that keep muscles slightly tense throughout the day begin to weaken, leading to a drop in muscle tone characteristic of deep sleep. This shift does not occur instantaneously. Instead, the two systems overlap for a short time. One widely accepted explanation for hypnic jerks is that during this overlapping phase, a random burst of activity in the motor pathways reaches the muscles before inhibition is fully established. This results in a sudden contraction, perceived as a fall or a stumble. Research published in Sleep Medicine in 2014 was among the first detailed studies describing the pattern of muscle activation during these jerks. A related study from 2016 showed that muscle activity does not spread systematically from one part of the body to another during such events. Researchers interpreted this as evidence that the signal originates deep within the brainstem rather than in the cerebral cortex. The sensation of falling is considered an intriguing aspect of this phenomenon. Among sleep researchers, it is widely accepted that the feeling of falling is not the cause of the jerk but rather a narrative constructed afterward. When the brain’s cortex is partially “disengaged,” it receives a strong motor signal that it cannot immediately interpret. It constructs a plausible explanation, falling, as a result. Another theory focuses on the vestibular system, responsible for balance and located in the inner ear. If muscle tone drops suddenly, the brain might temporarily misinterpret this change as the beginning of a real fall, triggering a reflex to correct posture. This is the same automatic mechanism that helps someone who trips over an edge regain their balance. This interpretation of a misinterpreted fall makes the hypothesis about life in trees particularly compelling. If someone were sleeping on a branch, the loss of muscle tone would not merely be metaphorical. A reflex that would jerk the body, check the support, and restore position before sleep fully takes over could indeed be useful. Such beneficial traits would have been retained through natural selection. Jason Ellis, a scientist studying sleep at Northumbria University, wrote in The Conversation that a hypnic jerk could allow a sleeper to check their support before entering unconsciousness, especially if they had fallen asleep on a tree. The origin of this behavior dates back to a time when primates spent millions of years in tree canopies, with many still doing so today. Monkeys sleep sitting on branches, relying on ischial tuberosities, firm, thick cushions on the buttocks, for support. Large ape-like primates go even further, constructing new nests almost every night from bent and interwoven branches. The habit of regularly sleeping on the ground is relatively recent in our evolutionary lineage and not the norm that early ancestors followed.