Darkness has long been one of humanity's greatest fears. It represents the unknown and undiscovered, but also danger and evil. Shrouded in darkness, one is unable to see their surroundings. This visual impairment often leaves people feeling vulnerable and frightened. Nyctophobia, aka fear of the dark, is a very common phobia that many people have. One person who many people don't know also had this fear was the famous inventor, Thomas Edison.
Edison's fear of the dark might at first come off as ironic. After all, he is the man that is often credited as the creator of the first light bulb. But then it makes sense because, if you're scared of the dark, why not just turn on (or in Edison's case "invent") the lights. However, what few people realize (in addition to Edison's fear of the dark) is that Edison did not actually invent the light bulb. In fact, Edison just hired a bunch of other inventors to work for him. Whatever they invented, he took credit for, which is how he managed to amass over a thousand patents in his lifetime.
Darkness is frightening not because of the vulnerability it induces but also because of all the unknown secrets it hides.