Diurnality is a form of plant and animal behavior characterized by activity during daytime, with a period of sleeping or other inactivity at night. The common adjective used for daytime activity is "diurnal". The timing of activity by an animal depends on a variety of environmental factors such as the temperature, the ability to gather food by sight, the risk of predation, and the time of year. Diurnality is a cycle of activity within a 24-hour period; cyclic activities called circadian rhythms are endogenous cycles not dependent on external cues or environmental factors except for a zeitgeber. Animals active during twilight are crepuscular, those active during the night are nocturnal and animals active at sporadic times during both night and day are cathemeral. Plants that open their flowers during the daytime are described as diurnal, while those that bloom during nighttime are nocturnal. The timing of flower opening is often related to the time at which preferred pollinators are foraging. For example, sunflowers open during the day to attract bees, whereas the night-blooming cereus opens at night to attract large sphinx moths. In animals Many types of animals are classified as being diurnal, meaning they are active during the day time and inactive or have periods of rest during the night time. Commonly classified diurnal animals include mammals, birds, and reptiles. Most primates are diurnal. Scientifically classifying diurnality within animals can be a challenge, apart from the obvious increased activity levels during the day time light. Evolution of diurnality Initially, most animals were diurnal, but adaptations that allowed some animals to become nocturnal is what helped contribute to the success of many, especially mammals. This evolutionary movement to nocturnality allowed them to better avoid predators and gain resources with less competition from other animals. This did come with some adaptations that mammals live with today. Vision has been one of the most greatly affected senses from switching back and forth from diurnality to nocturnality, and this can be seen using biological and physiological analysis of rod nuclei from primate eyes. This includes losing two of four cone opsins that assists in colour vision, making many mammals dichromats. When early primates converted back to diurnality, better vision that included trichromatic colour vision became very advantageous, making diurnality and colour vision adaptive traits of simiiformes, which includes humans. Studies using chromatin distribution analysis of rod nuclei from different simian eyes found that