Mammals

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.