There is an automatic gain control on the NVG tubes which effectively does the same as increasing or decreasing the ISO on your camera - when it is really dark, the gain control goes to max to amplify as much light as possible but you then get unwanted 'noise' speckling, honeycombing etc.
In a bright light, the gain control does the opposite - partly to prevent damage to the tubes - but you only have about a 40 degree field of view so you can look slightly away from the light source and keep a good picture.