It’s a funny thing actually. So, you are starting to fall asleep; your brain starts sending signals throught your body to prepare you to sleep. Your heartrate slows down, your breathing slows down, your body temperature starts decreasing. Your metabolism slows down. Sometimes, the same brain that sent signals to make these asjustments thinks “OH SHIT WE’RE DYING” and so sends a jolt of signals to wake you up.
I was once told this was called the hypnogogic jerk. The theory i was taught was that it was essentially random firing from the brain that tests if you’re fully paralyzed before sleep (your body is paralyzed when you are in light stages of sleep so you dont move while you dream, but not in deeper stages).
Kind of related is sleep paralysis where you become awake but your body is still paralyzed.
For me, it sometimes feels like the parts of my brain related to proprioception (knowing where your body is in space) fall asleep before the threat-identifying parts and conscious parts. The stillness of laying in bed causes a lack of sensory input, so my proprioception has no new information to process. This can end up feeling like I’m floating or falling, so my threat-identifying parts of my brain force me to make a sudden movement just to verify that I’m still grounded.
I’d guess that humans, being apes, are just particularly sensitive to the sensation of falling out of trees. Our brains are hard-wired to constantly watch for this danger, especially when sleeping (since our ancestors slept in trees), so we can perceive falling even when it isn’t happening if enough sensory input suggests that it could (like Richie’s Plank Experience in VR). I assume this hard-wired sensitivity to the possibility of falling is also what causes a fear of heights for many.
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