There are different levels of light and dark present throughout the day. Normally, one would expect to live in light during the day and in darkness at night. In total darkness, the color is not black because human eyes adjust to darkness, and thus the darkness is less pronounced. It also depends on outside light coming in through the window. An upset in this pattern has an effect on our biological rhythms. If you don't sleep in a room that is cool, quiet and dark, you may have sleep disturbances. Specifically,If you sleep in darkness, and wake up in darkness, your biological rhythm is interrupted, and you may feel tired. So, going to bed in darkness, with the eyes adjusting naturally, and waking up to light is the best way to get a good night's rest. Living in darkness is harmful to one's health. The body needs both light and dark - in the right amount.
Yes, it is normal human behavior to be afraid of the dark because it represents the unknown. The darkness hid predators and dangers when humans lived outside. This instinct lives on in humans today. It is completely natural to be afraid of the dark since it hides what is really there.
No, it is not normal human behavior to be afraid of the dark, but instead it is normal human behavior to be fearful when not able to see as well. Darkness itself is not what causes people to be afraid, but the prospect of not being fully aware of surroundings is what actually scares people.
Science shows that the color we see in total darkness is not black, but rather an extremely dark grey. In light of this, it's time to call into question the normalcy of being afraid of the dark. I think this is an ingrained societal myth, founded on stories that have been passed down for generations, rather than expected human behavior.