When you’re trying to sleep, you will probably have been jolted back awake sometimes. Kind of a pain really, isn’t it? Just about to nod off, and then – *JERK* – you’re back awake. Great.
This motion is called a hypnagogic jerk, an involuntary spasm of the muscles whilst you’re in a hypnagogic state. This is, as you may have guessed, the state you’re in between being awake and asleep. You’ll often find it’s what causes you to wallop your significant other during the night since it happens a fair few times while you’re dreaming.
Why does this happen though? It feels quite an extreme measure for the body to take, especially when you’re trying to sleep. It’s believed that factors such as stress, anxiety, fatigue or sleep deprivation could have ties. Another theory suggests it’s a misfire of nerves while you’re falling asleep, as your body thinks you’re dying. A more primitive theory is that it’s a form of primal reflex designed to stop us falling from trees.
I often found I experienced this jolting a lot during my early commute on the bus for my first job, back before colour was invented. Hopefully, nobody got kicked too hard.
Source: Live Science