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Diurnality is an animal behavior characterized by being active during the day and sleeping at night. Animals that are not diurnal might be nocturnal (active at night) or crepuscular (active primarily during twilight, i.e., at dusk and dawn). ^[1] Many animal species are diurnal, including many mammals, insects, lizards and birds. The diurnal pattern is often controlled internally by the circadian rhythm (endogenous rhythm) of the animal. In some animals, especially insects, external patterns of the environment control the activity (exogenous rhythms, as opposed to patterns inherent in the habitat).^[2]

Some mainly nocturnal or crepuscular animals have been domesticated as pets and have changed into diurnal animals to coincide with the cycle of human life. Examples are pet dogs and cats, which are derived from the wolf and the wild cat. However, these animals may exhibit their species' original behavior when they are born feral.^{[citation needed]}

Many plants are also diurnal or nocturnal, depending on the time period when the most effective pollinators, i.e., insects, visit the plant. For example, as most angiosperm species of flower are visited by many various insects, the flower adapts its phenology to the most effective pollinators in order to ensure proper reproduction and longevity of the species. Thus, the effectiveness of relative diurnal or nocturnal species of insects affects the diurnal or nocturnal nature of the plants they pollinate, causing in some instances an adjustment of the opening and closing cycles of the plants.^[3]

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