or maybe they just got used to sleeping with a light on, for those who don't know, (you), fear of the dark stems back to 40 to 10 thousand years ago when some people lived in the wild, predators that might eat them could have been hiding in the dark, so if there were no dark spots where predators could be, then they felt a sense of security, that fear is passed down to many of us which is why some people sleep with a light on.