Maybe monkeys or apes on the ground have thicker eyelids to block out the light? And do they sleep differently in general? http://www.bbc.com/earth/story/201504

Maybe monkeys or apes on the ground have thicker eyelids to block out the light? And do they sleep differently in general? http://www.bbc.com/earth/story/20150415-apes-reveal-sleep-secrets http://www.bbc.com/earth/story/20150415-apes-reveal-sleep-se... might shed some light (!) But I'd think that humans have always had shelter. Or is there evidence of communities sleeping out in the open?