An evolutionary pressure to become qualitatively more efficient in how we sleep therefore developed. Any Homo erectus capable of accomplishing more efficient sleep would likely have been favored in survival and selection. Evolution saw to it that our ancient form of sleep became somewhat shorter in duration, yet increased in intensity, especially by enriching the amount of REM sleep we packed into the night. In fact, as is so often the case with Mother Nature’s brilliance, the problem became part of the solution. In other words, the act of sleeping on solid ground, and not on a precarious tree
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