A day/night camera switches between a color image in good light and a monochrome image in low light, moving an infrared-blocking filter aside so it can use infrared illumination. The switch improves sensitivity after dark, and the trade is color information, which is often what makes an identification possible.
Night performance is where most camera specifications get stretched. The day/night mechanism is one real factor, and the infrared illuminator that usually goes with it is another.
The filter and the switch
In daylight an infrared-blocking filter keeps colors true. In low light the filter moves out of the path so the sensor can pick up infrared, and the camera produces a monochrome image. The transition follows a light threshold, and a poorly chosen threshold makes the camera switch back and forth at dusk.
Working with infrared
Infrared illumination reaches only as far as its rated distance, and it reflects off glass, rain and nearby walls. A camera mounted close to an overhang gets a bright wash across the frame. Check the scene at night with the illuminator on, because a daytime test proves nothing about the picture after dark.