A grainy image at night can be improved by lowering the gain settings on the video recorder or camera. Lowering the gain setting will also darken the image, however when using cameras with IRs the effect will be minimal. The gain setting is found within the color settings of the device. For more information on adjusting color settings, please refer to the Video steam is discolored (blue, green or red hue) article.
DH Vision suggests leaving gain on automatic for most applications, since for most night time scenes, no automatic gain control can cause the image to be dark and for all practical purposes, useless. As with shutter speed, fixing the gain is best in constant 24/7/365 lighting conditions. Capping the gain may be preferred to keep the gain from being set to high thereby causing noise.