Nocturnal creatures generally have highly developed senses of hearing, smell, and specially adapted eyesight. Such traits can help animals avoid predators. Some animals have eyes that can adapt to both low-level and bright day levels of illumination (see metaturnal). Others can function only at night. Many nocturnal creatures have large eyes in comparison to their body size in order to compensate for the lower light levels during the night. More specifically, these animals have been found to have a larger cornea relative to their eye size than diurnal creatures, in order to increase their visual sensitivity in the low light conditions.
While most sentients are diurnal, for various personal and social/cultural reasons some people are temporarily or habitually nocturnal.