system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do people get that eerie feeling that something's under the bed when they're younger?

Or, similarly, that feeling like there's someone behind you after you turn the lights out in the basement and sprint up the stairs.
gpt
For most of our evolutionary history, the thing that was most likely to kill us was other, bigger animals intent on making us dinner. It's only been relatively recently that we've had big enough brains to rely upon fires and weapons and big walls to keep us safe from hungry carnivores. As far as our subconscious is concerned, we are still an animal that could be preyed upon.

Now, as far as our instincts are concerned, there are certain times when we're pretty safe, such as when we're among a large group of our own kind, and when it's broad daylight and can see anything that might be about to "get" us. But when we're alone in the dark, we are theoretically in a high-risk situation. Oh, and if we happen to be a child, our vulnerability is of course even greater. As rational human beings we may be aware that it's unlikely there is a sabre-toothed tiger with superb night-vision and stalking skills about to pounce on us, but our evolved subconscious still thinks it's a distinct possibility.

So, how does our brain respond to this? By putting us on extra high alert mode. It pushes forward into our consciousness the idea that there *might* be something lurking there. It makes us ready to run. We have probably all been in a "scary" situation, alone at night time, when we suddenly start thinking about monsters lurking in the shadows, and all of a sudden the slightest innocent noise put us straight into panic mode and we go running to safety. This instinct would have saved the lives of our ancestors on countless occasions.

Also, this is why if we are not in a position to run away we get the urge to hiding entirely under the covers, meaning any imaginary prey animals cannot see us.