Historically, most armed conflicts with criminals have taken place under less than optimum light conditions. At least two thirds of all police action shootings take place in poor light and I would expect that events involving armed citizens fall along the same lines. To get right to the point, the bad guys continue to use the cloak of darkness to conceal their illicit activities. Your chance of being the intended victim or witness to a crime is greater at night than during the day.
Humans are visual creatures and most of the information we process is done through our eyes. Darkness, however, plays havoc with this ability to take in and process information. Location and assessment of potential threats becomes increasingly difficult as light levels fall. Likewise, if action is required, everything from empty hand skills to efficient use of a firearm is far more challenging. Poor light often masks an adversary’s intentions to the last second and if deadly force is justified, sights, target, and background are difficult if not impossible to define.