Staying up all night could be in our genes - a group of Chinese scientists has found new evidence to suggest a common ancestor of humans and apes was probably nocturnal.
The group's study into the evolutionary history of primates found that about 50 million years ago, some primate ancestors abandoned the shadow of night in favour of the bright light of day, acquiring new capabilities, such as sharper eyes, that increased their competitiveness.
The scientists believe an ancient line of haplorhines - 'dry-nosed' primates - experienced an evolutionary "great leap forward" in visual capability that enabled the switch and led to today's primates.
They say the rapid change suggests that early primates shifted their active hours, as sharper eyes helped them find food and evade predators in daylight.
Dr Wang Guodong, associate researcher with Chinese Academy of Sciences' Kunming Institute of Zoology and an author of the paper, said the findings helped to "set a correct bioclock" for studies on early primates.
"Nocturnal creatures relied mainly on non-sighting senses, such as nose and ears, for nighttime activities, but we have not detected major genetic evolution events on these senses.
"The evolution of sight-related genes was rapid and profound, which convinced us that the transition from nocturnal to diurnal must have happened."