While winding down to sleep, sometimes people startle themselves (and their bedmates) with a massive, involuntarily jerk. Here’s the science behind this curious behavior–and why you may often dream of falling. From The Secret Life of Sleep.
The scientific name for this phenomenon is myoclonic kick, though it is more commonly known as a sleep start. While it appears to be a primitive reflex generated by the brain stem, no one really knows why we startle like this on the cusp of sleep. Some theorize it is because our brains mistakenly confuse the sudden release of muscle tension, especially among the muscle groups that resist gravity, with a frightening free fall and jerk to stop it, as if grabbing for a branch when tumbling from a tree. Others propose it is left over from the evolutionary need to keep watch for danger, waking us whenever we start to slip off.
Whatever the case, the sensation of dropping suddenly is so strong, people often have split-second dreams of falling, whether it is tripping while running, slipping on ice, tumbling down stairs, or plunging from a precipice. Neurologist Oliver Sacks finds these split-second dreams, which he prefers to call hallucinations, particularly fascinating because they demonstrate how quickly our minds can work to invent a narrative to explain a sensation. In the moment, it seems that the dream has triggered the startle, but it is probably the other way around. Sacks proposes that a “preconscious perception” of the jerk prompts “an elaborate restructuring of time” to provide a story for the event. If he is right, and I suspect he is, our startle dreams are actually cover stories, something like the tales kids spin to explain why the cookies are gone, only more convincing.
These tripping, slipping, tumbling, and plunging dream stories are so common, and so convincing, they have made it into more than a few lullabies. My favorite is the popular “Rock-a-Bye Baby,” which has the baby blowing in the wind and falling “cradle and all” at the end. I have always wondered why a lullaby intended to lull an infant to sleep would include such frightening imagery; it seems a cruel thing to give a child heading into the land of Nod. Now I understand. Bedtime is falling time, and these lullabies prepare us for the hallucinations of falling that so often attend the slip into sleep.