Morbid fear of the night and darkness.

Nyctophobia (from Greek νυξ nic-to-pho-bi-a: "night" and phobia, also called scotophobia, from σκότος - "darkness", lygophobia, from lyge - "twilight", or achluophobia or noctiphobia) is a pathological fear of the dark. Its symptoms include breathlessness, excessive sweating, nausea, dry mouth, feeling sick, shaking, heart palpitations, inability to speak or think clearly or sensation of detachment from reality. Normally, since humans are not nocturnal by nature, humans are usually a bit more cautious or alert than in the day, since the dark is a vastly different environment.