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For six nights in a remote corner of Senegal, Fiona Stewart slept like a chimpanzee, snoozing in abandoned nests of woven branches some 30 feet off the ground. In a scientific sense, her endeavor was a success — compared with sleeping on the jungle floor, the University of Cambridge anthropologist recorded fewer bug bites and felt demonstrably safer from predators such as hyenas, as she wrote in the American Journal Of Physical Anthropology.

As far as getting a good night’s rest? That was a different story.

“It was exhausting,” Stewart says. “We are too long for the typical chimpanzee nest (even me being only five foot two inches).”

She was right to complain: The last time someone from the genus Homo was able to sleep comfortably in a nest, we didn’t grow much taller than four feet. 

Travel back three million years, a few hundred millennia before evolution pulled the twist that is Homo, and you won’t find any little hominids sleeping on the ground or in caves. According to Frederick Coolidge, a psychologist at the University of Colorado Colorado Springs, he’ll tell you that although Lucy, that upright ambulating ape-like species known as Australopithecus afarensis, hopped down from branches to fuel her bigger brain by hunting bush babies, she was primarily a tree-dwelling creature.

“Lucy still played in trees, probably mated in trees, probably had their babies in a nest,” he says. Lucy also had a “spider-like” body. Considering her long arms and long limbs, her proportions would resemble the gangly monkeys who swing through trees today.

“Without a doubt she slept in the nest, in a tree, probably 45 feet high,” Coolidge says.

Couple innovation and better planning with large communicative groups, and you have a creature primed to conquer continents.

After Lucy came Homo habilis — who, for the record, Coolidge believes was not a direct ancestor of humans and would more properly be called Australopithecus habilis. It would be 1.8 million or so years ago until tall, strapping Homo erectus would stroll on to the scene. Coolidge believes erectus was the first ground-sleeper.

Stewart, however, is a bit more cautious. She says that while we’re able to surmise that ground sleeping didn’t happen until Homo erectus, it's difficult to know from fossil records. Some early primates, she argues, had tree-living traits but didn’t seem to exhibit such behavior, just as modern humans don’t have tree-dwelling traits yet we’re quite adept at climbing trees.

From Coolidge’s point of view, erectus’ body size sealed their sleeping fate. An adult Homo erectus grew to six foot, one inch; Coolidge cites the erectus specimen Nariokotome boy who, at an estimated age of eight, appears to have been well over five-and-a-half feet tall.

“Erectus,” he says, was “way too heavy to sleep in a tree.”

With the transition to the ground, so the thinking goes, came social and mental changes. In a 2006 paper, Coolidge and an UCCS anthropologist named Thomas Wynn outlined how sleeping on the ground meant longer periods of uninterrupted REM and slow-wave sleep. Homo erectus, therefore, had better dream recall of both creative and threatening dreams.

“We are too long for the typical chimpanzee nest.”

Primatologists Gottfried Hohmann and Barbara Fruth have proposed a similar cognitive change with the building of nests, Stewart points out.

“A second leap with the transition to ground sleep seems likely,” she says, “and if it coincided with the use of fire, there was another influence in creative bursts due to possible changes in diet and sociality.”

Throughout human history, there’s no shortage of dreams sparking innovation (think Dmitri Mendeleev and the periodic table). Among Coolidge’s theories — Homo erectus’ beautiful stone axes were born in dreams. According to Coolidge, the stone tools of habilis were crude, nothing more than a flake off a rock.

“You could use the core to break nuts, and the flake to cut meat,” he says.

Compare that with Homo erectus’ “nigh near-perfect stone handaxes: symmetrical and sharp on all the edges, a sinuous edge, and a sharp point — a true Swiss Army knife.”

How did this happen? Coolidge echoes Antti Revonsuo, the Finnish dream psychologist who reported that two-thirds of dreams have an antagonistic component.

“Imagine if you dreamt that you were naked, which is a very common dream,” Coolidge says, “and you’re out on the prairie, and the tigers are coming and it’s raining. You're going, ‘Where’s my extra coat? Where’s my hand-axe? I forgot it, and now I'm naked.’ You wake up from that dream and if you’re planning to go out on the prairie, I guaran-damn-tee you, you brought your extra coat and you brought your extra hand-axe.”

Couple innovation and better planning with large communicative groups, and you have a creature primed to conquer continents.

While it's true we’ve come quite a way since our tree-sleeping habits, we’ve likely retained at least one physiological feature that arose when we nodded off in nests: The involuntary twitch upon falling asleep known as a hypnic jerk.

Cooldige won’t say for sure why the hypnic jerk exists, but he has his theories. If you’re Lucy, dead tired after a long day of hunting bush babies, before you can totally pass out the jerk kicks you awake. High up off the ground, “you would readjust your sleeping position and you make sure you’re in the middle of the nest and not at the edge.”