The most electrifying study by far was by the UCLA sleep researcher Jerome Siegel and his colleagues, who affixed wearable sensors to ten Hadza hunter-gatherers from Tanzania, thirty San forager-farmers from the Kalahari Desert, and fifty-four hunter-farmers from the Amazon rain forest in Bolivia. None of these populations have electric lights, let alone clocks or internet access. Yet to Siegel’s astonishment, they slept less than industrialized people did. In warmer months, these foragers slept on average 5.7 to 6.5 hours a day, and during colder months they slept on average 6.6 to 7.1 hours
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