Nyctophobia is an extreme and irrational fear of darkness. Being afraid of the dark is quite common in children but nyctophobia can affect adults to varying degrees of severity and lead to disruptions in their lives.
For instance, sufferers of this may not even be able to go outside at night or turn off lights because of their fear. This fear most likely evolved in humans due to the reduction in vision that makes dangers and predators harder to see and escape from. Cognitive-behavioral therapy is very effective for this phobia and is effective in treating around 90% of patients.