Frightful Minded

Riddle: I get in the minds of children, the minds of men, although they can always never know when, they fear me when I creep at night, in their heads I create a fright. What am I?

Answer: A nightmare. We all have one sometimes.

Source: https://www.riddles.com/5245

I get in the minds of children, the minds of men, although they can always never know when, they fear me when I creep at night, in their heads I create a fright. What am I? — Frightful Minded by Caleb Choi Riddle