The<unk>  is a common<unk>  or phobia among children and, to a varying degree, of adults.<unk>  is usually not<unk>  of<unk> ness itself, but<unk>  of possible or imagined dangers concealed by<unk> ness. Some degree of<unk>  is natural, especially as a phase of child development. Most observers report that<unk>  seldom appears before the age of 2 years. When<unk>  reaches a degree that is severe enough to be considered pathological, it is sometimes called achluophobia, nyctophobia (from Greek νυξ, "night"), scotophobia (from σκότος - "darkness"), or lygophobia (from λυγή - "twilight").
Some researchers, beginning with Sigmund Freud, consider the<unk>  as a manifestation of separation anxiety disorder.
An alternate theory was posited in the 1960s, when scientists conducted experiments in a search for molecules responsible for memory. In one experiment, rats, normally nocturnal animals, were conditioned to<unk>  the<unk>  and a substance called "scotophobin" was supposedly extracted from the rats' brains; this substance was claimed to be responsible for remembering this<unk> . Subsequently, these findings were debunked.
Fear of the dark
Jealousy
Brain fog
Unconsciousness
Sadness
Somatization
Blunted affect
Blindsight