Fear of the dark is common and an almost universal fear for at least a part of our lives (usually some part of our childhood). Though it's not so much the dark that we fear, but the unknown. There is an inner sense of danger that brings it to the conscious mind. Things one cannot see can hurt you. This trait or human characteristic is from early man's development, when mankind was not the top predator in his environment.
As humans mature, we internalize the real odds of something dangerous hiding in the dark, and that calms us in appropriate situations, but that fear can come back when we are in unfamiliar places. The reason we are all afraid of the dark at times is because that fear gave us an advantage during our evolutionary history. Since man relies upon his senses for survival, taking away one of them (sight) because of darkness would naturally lead to anxiety about what could be out there in the dark.
Ancient man quickly learned that many predators prefer the cover of darkness to hunt and over time that association strengthened into a subconscious absolute: stay out of the dark because that's where the danger is.
While fear of the dark can manifest itself as an acute reaction—like panicked screaming when someone suddenly turns out the lights, or as insomnia, as a recent study conducted at the University of Toronto suggests—it more commonly manifests as foreboding anxiety. The emotion of anxiety plays a specific role in our behavioral responses to stimuli just as the emotions of love, anger, and sadness do, acting to increase our ability to cope with stress and more fully exploit beneficial opportunities.
Each emotion can be thought of as a computer program designed to accomplish some specific fitness task particularly well. If the current task is courtship, romantic love is helpful. If one is being betrayed, anger is useful. If a tiger is attacking, then fearful flight and avoidance are best. If people are disapproving, then social anxiety may be appropriate. Different emotions, however, must be orchestrated, just as endocrine function must be coordinated in an endocrine orchestra. Emotional responses must fit changing adaptive challenges, with each emotion fitting a particular kind of situation.
Specifically, anxiety increases your awareness of situations that may damage your reproductive resources. Not just your vital signs, but your relationships, income, social standing, physical features; anything that makes you more attractive to the opposite sex and more likely to reproduce.
Unlike anger or sadness, which occur in direct response to a specific event, the study showed responses to environmental cues indicating a potential future loss with anxiety prior to the expected event, since that's when it is most evolutionarily beneficial. Our ancestors who best recognized and responded to these cues increased their chances of survival and reproduction compared to their more less-aware peers.
Since that response is correlated with natural selection, certain cues—such as snake- or cat-eye patterns, spider-like objects, and darkness (where the snakes, leopards, and spiders all live)—more easily invoke that emotion. This is the result of generation upon generation of early humans reacting to and subsequently preparing for dangers such as these. That's why many ancient threats—uncontrolled fire, spiders, snakes, predators, and darkness—are more easily instigate a negative response, even in small children, than modern threats like automobiles, nuclear war, or guns do.
While anxiety is deeply rooted in our psyche, it is not entirely instinctive. We also learn fear and anxiety responses from our parents. If a small child is frightened of an new or unknown object and its mother responds in a calming manner, the kid learns that the item is not a threat. If the parent responds with apprehension herself, however, the child's fears are confirmed and enhanced. This allows for offspring to rapidly learn of the dangers around themselves without actually having to be bitten by snakes, mauled by lions, or be electrocuted by a penny in the wall socket while simultaneously figuring out that everything from leaves and small rocks to thunder and scary scenes on TV aren't actual threats.
It is a system that is based on experience and transfer of knowledge, but triggers an instinctive response. This subtle gnawing emotion has been honed and refined over millennia by both nature and nurture into a vital survival response that remains as useful in the modern world as it did in the Neolithic Period of Humanity. As environmental cues have changed over time, anxiety over social interactions and property such as finding shelter and not starving to death, remains the same. Humans have the inner voice that tells us to keep safe so you can live to reproduce another day.
The fear of the dark, and by extension the fear of the unknown, are hard wired tools to make sure humans don't ever forget the basic purpose of all human beings. Fear itself is a survival mechanism.
