system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do I sometimes get this weird sensation like I'm falling through space when sleeping?
gpt
Looks like there are a few ideas about this. It's part of a Hypnic or Hypnogogic jerk which happens in non-REM sleep and is often accompanied by a falling sensation. And people think it could be caused by anything from anxiety to caffeine. It happens more often in young people and people with irregular sleep schedules.     
    
Beyond that we really don't know much about it. But there are at least a couple theories about why it has evolved.     
   
One idea is that it's just a natural part of your body shutting down for the night and nerves misfire a bit in the process.    
This idea makes sense.    
     
Another idea is that has to do with a primate ancestry. The idea is that the brain thinks it is falling out of a tree.    
This idea makes no sense.