The nocturnal bottleneck hypothesis is a hypothesis to explain several mammalian traits. In 1942, Gordon Lynn Walls described this concept which states that placental mammals were mainly or even exclusively nocturnal through most of their evolutionary history, from their origin 225 million years ago to after the Cretaceous–Paleogene extinction event, 66 million years ago. While some mammal groups later adapted to diurnal (daytime) lifestyles to fill newly unoccupied niches, the approximately 160 million years spent as nocturnal animals has left a lasting legacy on basal mammalian anatomy and physiology, and most mammals are still nocturnal.
Artist's impression of the Purbeck lagoon at dusk: Durlstotherium (right and center foreground) and Durlstodon (left foreground) ventured out at night to hunt insects. The theropod Nuthetes holding a captured Durlstotherium (centre middle distance).
The tapetum lucidum of a European badger reflects the photographer's flash, one of many nocturnal traits ubiquitous in mammals
Brasilitherium, a very advanced near-mammalian cynodont, were probably nocturnal burrowers.
The whiskers on a shrew, used in finding prey, navigation and socialization
Nocturnality is a behavior in some non-human animals characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnal meaning the opposite.
Owls are well known for being nocturnal, but some owls are active during the day.
The kiwi is a family of nocturnal birds endemic to New Zealand.
The honey badger, a nocturnal animal.
Light pollution on a ski slope in Finland gives the area a hazy, brightened sky.
