You have two different kinds of light receptors in your eye, called rods and cones. The cones are for brighter light conditions and allow you to see in color. The rods allow you to see in low light conditions, in shades of gray. Sometimes and for unknown reasons your rods will shut down making it appear that the room has grown very dark. Although you can still pick out colors like from a green night light etc., your overall vision can seem to become very dim. Sometimes this can happen with just one eye at a time making it look dark in only one eye. Chances are the perceived change in brightness could be do to the light receptors in your eyes adjusting to stimuli.