Is Elon Musk An "Absolute Hero...
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For more on marking an answer as the "Best Answer", please visit our FAQ.We have adapted to be able to shift position to prevent cramps and stiffness in muscles which could be need at short notice.
In prehistoric man, the need to wake instantly and be alert to fight or run was vital to survival - muscles needed to have adequate blood supply, and limbs not cramped or 'gone to sleep'. This all needed to be arranged withut disturbing sleep, so the body evolved to carry out these tasks, while letting the brain continue to rest.