Whether real, imagined, or supernatural, people have had scary experiences without anybody around to validate them. Thankfully, that's what the internet is for.
People shared the creepiest things they'd experienced when they were alone, and they're way more frightening than just plain loneliness.
When I was a kid, I used to spend a lot of time alone in my basement. We had a TV down there, so I was down there pretty late most nights in the summer. One pretty late night, I swear I heard rustling from behind me, but when I turned around, there was nothing there. I felt a really sharp breath on the back of my neck (hard enough to blow my hair). Took off up the steps, leaving everything on downstairs. I grabbed my mom and we went back downstairs to check it out. Everything was turned off.
I didn’t hangout downstairs anymore.