Crepuscular, diurnal, and nocturnal are terms which are used to describe the period of the day during which an animal is active. Many animals develop distinctive behavioral patterns which are part of their overall adaptation to the environments that they live in. Periods of activity tend to be easy for biologists to observe, and they are commonly used as tools to assess whether or not animals are stressed, frustrated, or pressured into unusual behavior. Knowledge about when an animal is supposed to be active can also be used as an indicator to suggest that an animal might be sick; owls, for example, are usually seen at night, making an owl a cause for concern if it is seen at noon.
When an animal is said to be crepuscular, it is active during the twilight hours at dawn and dusk. The word "crepuscular" is derived from a Latin word which means "twilight." Many animals take advantage of the twilight to feed, seek out water, and engage in other behaviors because they know that predators are not as active in the twilight. Visibility is also challenging at dawn and dusk, making it easier for animals to hide from potential threats. Rabbits and cats are both famously crepuscular.
Nocturnal animals, on the other hand, only come out at night. Many of these animals have a specially developed sense of vision which helps them to see in the dark, and they often have excellent hearing as well. There are a number of reasons for an animal to be nocturnal; many desert animals, for example, are active at night because it is cooler, and their rate of water loss is decreased as a result. Two famous nocturnal animals are bats and owls.
Animals who are active during the day are known as diurnal. Like crepuscular and nocturnal animals, the daily patterns of diurnal animals are established through circadian rhythms, patterns built into their body chemistry. Being active during the day certainly has some advantages, especially for animals with poor visibility like humans. Humans also have a tendency to encourage diurnal behavior among the animals they interact with; dogs, for example, are naturally crepuscular, but they learn to become diurnal through human interactions.
A disruption of an animal's established biological rhythms can be harmful. Nocturnal animals, for example, tend to feel very disoriented when they are forced to be active during the day, and this can cause a feeling of stress and unhappiness which can have a negative impact on the animal's health. Diurnal animals are not adapted to being active at night, which means that they can be vulnerable to predation when they are out and about after dark, while crepuscular animals like deer can feel very stressed when they are out in the heat of the day or late at night, and this can cause them to engage in odd behaviors like running into traffic.