We stopped checking for monsters under our bed when we realized they were inside us.
As a kid I used to be scared of the night watchman. Every night, during his patrol, as he passed by my house, creating a ruckus with his baton and whistle, I used to curl up into a bundle of pure fear. I could never dare to look out my window to invalidate my baseless imaginations but my young mind painted the image of a red-eyed hairy daemon, walking through the streets in search of kids to devour. The quiet of the night and the solitude used to make the moments all the more ghastly. I knew he was employed as a guard and it was his duty to keep thieves and burglars off our houses, but the knowledge never helped me feel otherwise.
As a kid, what frightened me most were imaginary things that I believed were real. As an adult, I realize how easy I had it back then. What keeps me awake at night now is the fear that what I believe is real might just be my imagination.