Our night vision depends on photoreceptive structures in our eyes called "rods" that have a pigment called rhodopsin which is sensitive enough to be triggered by as little as a single photon of light under ideal conditions.
Unfortunately our rods have a saturation point, a point at which they essentially white out and our cones (the structures of our eye responsible for vision in daylight and perceiving color) take over.
If our rods are exposed to too much light they become essentially useless for a brief period of time, and it takes as long as 30 minutes for them to "recharge" to the point where you regain your peak night vision capabilities.
All of us have at some point gone from a place where there is abundant light into a place that has almost no light and we've all found out that it takes us a couple of minutes in the dark for our eyes to adjust so we can actually have some idea of what the environment around us is like.
Some human beings have learned to use this shortcoming we all share to their advantage. Statistics tell us that most violent crimes happen at night or in conditions of low light. A sizeable majority of officer involved shootings happen at night. Bad guys seem to like the cover of darkness and often use it to prey on their fellow man.
Men who make preying upon their fellow man a lifestyle look at the world much differently than normal people do. They view darkness as an asset. They use the cloak of darkness as a weapon against those they would victimize. Knowing this you should now understand why the statistics show that bad guys do most of their work at night.
The defense minded individual should also notice that these realities mean that if they are forced to defend themselves odds are it will be under conditions of low light.
...so what do they do about it?
Thankfully there are some options.