>In our past we used to sleep in groups and so at every point in time, at least one group member was alert to watch out for predators This is also the reason b

>In our past we used to sleep in groups and so at every point in time, at least one group member was alert to watch out for predators This is also the reason behind evolutionary pressure to have a certain percentage of night owls among the tribe who naturally sleep late and act as eyes and ears against predators.