As a species, we have never liked the darkness of night. For several hundred thousand years we have retreated to caves and other shelters, huddled closer to blazing fires, and shouldered a heavier burden of anxiety soon after sunset. Predators real and imagined lurked in the shadows; denizens of the spirit world had their way after dark. Everyone had stories of bad things that happened when the vital sense of vision was rendered impotent at night.