Light can produce powerful masking effects on an animal's circadian rhythm, meaning that it can "mask" or influence the internal clock, changing the activity patterns of an animal, either temporarily or over the long term if exposed to enough light over a long period of time. Masking can be referred to either as positive masking or negative masking, with it either increasing an diurnal animals activity or decreasing a nocturnal animal's activity, respectively. This can be depicted when exposing different types of rodents to the same photoperiods. When a diurnal Nile grass rat and nocturnal mouse are exposed to the same photoperiod and light intensity, increased activity occurred within the grass rat (positive masking), and decreased activity within the mouse (negative masking).