The element of the unknown is very effective. For example, in a short creepy story, if there's a strange noise but you don't know what's causing it, it creates fear. It makes the reader or listener imagine the worst - like a monster or a ghost.
Unnatural or distorted appearances can make a short creepy bedtime story effective. Imagine a creature with too many eyes or limbs that moves in a jerky, unnatural way. Also, a setting that is dark and foreboding, such as a fog - covered cemetery or a dilapidated attic, adds to the creepiness. And when there's a sudden, unexpected change, like a normal - looking object suddenly coming to life, it catches the reader off - guard and makes the story more frightening.
A sense of isolation also works well. When the character in the story is alone, like in a big, empty house at night, it intensifies the creepiness. There's no one to turn to for help. Another important aspect is the use of sound. Strange sounds like creaking floors or whispering voices can really send chills. For instance, in a story where the only sound is a slow, rhythmic dripping in an otherwise silent house, it can be really terrifying.