Mice are nocturnal by nature, so they generally forage by night and avoid daylight. That is why you hear house mice scurrying in the rafters or pet mice clattering round their cage while you are trying to sleep, and why predators like owls, cats and foxes get busy after dark. Scientists ask how such behavior patterns work – for example, how light and dark depress or stimulate mouse activity levels. Understanding how light affects mice may help us understand how it affects humans too. It may also suggest ways to help keep wild mice out of your home and garden.
How Mice Perceive Light
The eyes of nocturnal creatures have evolved differently from ours to enable them to see in the dark or make the most of very dim light. Mice also respond to ultraviolet light. Scientists have observed that mice with visual defects react to light even if they cannot distinguish particular images. They think that special cells in a mouse's eye contain a light-detecting protein called melanopsin, which sends messages to the brain about the degree of brightness.
Aligning activity to the daily cycle of alternating light and dark, work and rest, is essential to the survival of most organisms, including mice and humans. Circadian rhythms set the timing for sleeping and waking, and for the daily cycles of metabolism, hormone secretion and other functions. The rhythms are controlled by "clock" genes, which mainly respond to light. Researchers investigated this response in mice by exposing them to varying sequences of light and dark. They found that the circadian rhythms governing sleep adapted effectively. However, light delivered in continually alternating pulses made the mice sluggish and unable to learn.
Effects of Light at Night
Exposure to light at night disrupts normal metabolism in laboratory mice. Under light, they eat at times they normally would not be eating, and consequently gain weight. Some studies suggest that light at night may inhibit activities like wheel-running in laboratory mice, but results are not consistent, and night-lights seldom give pet mice any such inhibitions about running round their cage.
Researchers explain that mice often behave more aggressively in the dark nights of winter than in the light days of summer because light activates the hormone estrogen, which helps to curb aggression.
Light as a Mouse Deterrent
Artificial outdoor lights, such as bug lights and sodium street lights, appear to deter beach mice from foraging around the lit area at night. This may please people holding beach barbecues, but removing mice from the natural food chain can disrupt the local ecology.
Electronic devices are available that claim to scare rodents and other animals by emitting bright LED flashes or strobe lights. Gardeners sometimes use glittering strips of tin foil to try and scare mice off fruit and vegetable crops.