Nyctophobia (or noctophobia) is a phobia characterized by a severe fear of the night. It is triggered by the brain's disfigured perception of what would, or could, happen when in a night-time environment. It can also be temporarily triggered if the mind is unsteady or scared about recent events or ideas, or a partaking in content the brain considers a threat (examples could include indulging in horror content, witnessing vulgar actions, or having linked dark environments to prior events or ideas that disturb the mind). Normally, since humans are not nocturnal by nature, they are usually a bit more cautious or alert at night than in the day, since the dark is a vastly different environment.

Nyctophobia produces symptoms beyond the normal instinctive parameters, such as breathlessness, excessive sweating, nausea, dry mouth, feeling sick, shaking, heart palpitations, inability to speak or think clearly or sensation of detachment from reality and death. Nyctophobia can be severely detrimental physically and mentally if these symptoms are not resolved.

Nyctophobia may also be tied to nocturnal creatures, whether fictional or real. For instance, someone who experiences sanguivoriphobia, a fear of vampires, might also experience nyctophobia due to an association with vampires. Similarly, someone with chiroptophobia, or fear of bats, might also likewise have nyctophobia due to their association with the night or dark spaces.

More Info: en.m.wikipedia.org