Parents in hunter-gatherer societies know why kids don’t like to retire alone to a dark room at night, writes Peter Gray, a psychologist:
Until a mere 10,000 years ago we were all hunter-gatherers. We all lived in a world where any young child, alone, in the dark, would have been a tasty snack for nighttime predators. The monsters under the bed or in the closet were real ones, prowling in the jungle or savannah, sniffing around, not far from the band’s encampment. A grass hut was not protection, but the close proximity of an adult, preferably many adults, was protection. In the history of our species, infants and young children who grew frightened and cried out to elicit adult attention when left alone at night were more likely to survive to pass on their genes to future generations than were children who placidly accepted their fate. In a hunter-gatherer culture only a crazy person or an extremely negligent person would leave a small child alone at night, and at the slightest protest from the child, some adult would come to the rescue.
Safer than sleeping alone on the ground
h/t to Amy Alkon
Paleo Diabetic 
It’s also probably why we’re not well suited to sleeping soundly through the entire night. There’d be a selection balance between too-heavy sleepers (get eaten) and too light sleepers (never well rested). The notion that humans are ‘supposed’ to be happy and well rested and content does jive evolutionarily speaking. Those well rested and content ancestors would be rapidly conquered by the more aggressive and cranky types.
I hadn’t thought of that, Isaac. You may be on to something. I may try that explanation on my insomniacs.
-Steve