It is thought to be a safety system from our ape like ancestors. The system is designed to rapidly wake us, and make us latch on, if we start to fall. If you’re sleeping in a tree, this could be quite a critical response.

Unfortunately, sometimes, as we are falling asleep, the brain shuts down the senses etc in the wrong order. As our sense of balance shuts down, we can momentarily feel like we are falling. This triggers the previous response. Our muscles twitch fire, to try and latch on to something, along with a burst of adrenaline to get us awake. This is obviously less than ideal, if you are trying to get to sleep, in a nice warm bed.

I believe there are genetic components, but disrupted sleep is the most common cause. Overtiredness, disrupted circadian rhythms, or sleep disrupters (bright or blue light, caffeine alcohol etc) can all make it worse. If they are excessively common, they could be a symptom of a larger, more serious sleep condition (e.g. sleep apnea). Don’t stress about them, but don’t ignore them either, if they are disruptively common.