Most mammals are nocturnal. Studies have found that
many small mammals (for example, mice) eat less food in areas that
are lit by artificial light, assumedly to avoid predators.
Conversely, other studies have found that predators of small
mammals (for example, foxes), are attracted to lit areas, possibly
for easy prey. Artificial light has also been shown to affect the
circadian rhythm of some mammals, extending the day of diurnal
species, and shortening the day of some nocturnal species. In rats,
artificial light at night suppressed melatonin production, and
resulted in an increased rate of tumors.
Bats are well known to be affected by artificial
lights. Many species of bats use artificially lit areas as an easy
foraging ground, which can affect the local population of insects.
Some bats, however, avoid the lit areas, and are then outcompeted
by the bats that get increased food from the lit areas.