Why does my body 'jerk' when falling asleep?

A staggering 70 per cent of people have experienced the "fake fall" after finally drifting off to sleep – yet, people still don't fully understand what they are or how they happen.

The question was raised on Twitter, with one asking: "Don’t you hate it when you’re just drifting off to sleep and then your whole bodyjumps and you experience some kind of mini heart attack".

Another confused user wrote: "Lying in bed, drifting off to sleep and your whole body jerks you awake! What the hell is that all about?"

Meanwhile, a third asked: "Am I the only one that has random body jerks when they sleep?"

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So, what exactly are they?

The harmless and relatively common feeling is known as a "hypnic jerk" or a "sleep start".

It's when the body experiences "sudden muscle contractions" similar to when it "jumps" when scared. It has also been likened to a falling sensation.

Researchers don't actually know for sure what causes hypnic jerks, but they have several theories.

"When you fall asleep, researchers suspect that a misfire sometimes occurs between nerves in the reticular brainstem, creating a reaction that leads to a hypnic jerk," Sleep Foundation writes.

"For example, it may be that when your muscles relax completely, even though that’s a normal part of falling asleep, your brain mistakenly assumes you are really falling and reacts by twitching your muscles. It’s also possible that hypnic jerks are a physical reaction to the dream-like imagery that accompanies them."

The sleep experts suggested that excessive caffeine, stimulant consumption, vigorous exercise before sleep, emotional stress, and sleep deprivation could increase the likelihood of experiencing a hypnic jerk.

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