Just as an aside, but various animals see in "different wavelengths" all the time. Humans can see in a range of wavelengths which we call 'visible light'. When most mammals we know of allow our eyes to dark-adapt, what's happening is NOT that we are seeing in different wavelengths, but that we are allowing our iris to dilate completely and allowing the nerve endings triggered by the stimulus of the rods and cones to notice less-powerful stimuli, in the same fashion that after a few minutes in a silent room, our ears hear much fainter sounds than we would notice in the middle of a crowded subway station...And that we would miss if we had JUST stepped into that room FROM the crowded subway station. It's basically attunement, resetting the 'noise level' that we can single out and pay attention to.<br/><br/>If D&D darkvision works by adding wavelengths beyond human-visible light (i.e., infrared or ultraviolet) then those would just be 'other visible colors' to that person's eyes...And in an area without much UV or IR they would, after a few minutes, become attuned to noticing the faintest hint of those regions of the spectrum. There's no reason that light would cause that to 'wash out' any more than a bright red light would prevent you from noticing blue or black or yellow objects.<br/><br/>Alternately, it's simply magic...