Nocturnal bottleneck

The nocturnal bottleneck hypothesis is an evolutionary biology hypothesis, introduced by Gordon Lynn Walls in 1942, proposing that placental mammals were mainly or exclusively nocturnal for most of their evolutionary history, from their origin around 225 million years ago in the Late Triassic until after the Cretaceous-Paleogene extinction event 66 million years ago. Although some mammal lineages later adapted to daytime activity once the non-avian dinosaurs had died out, roughly 160 million years spent as nocturnal animals left a lasting imprint on basic mammalian anatomy and physiology, and most mammal species remain nocturnal today even though some groups have since become active by day.