system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is happening when you're almost asleep and you suddenly get that strange falling sensation?

It freaks me out! Why do our brains do that?
gpt
No one knows for sure. It's called a hypnogogic jerk. 

Best theory I've heard is that, as you fall asleep, your brain starts to ignore outside sensations (sight, sound, touch). But another part of your brain notices that there is no input coming in (especially touch input) and interprets that as falling.