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The Relationship Between Your Body's Internal Time and the External World
The Sentinel hypothesis proposes that in group living settings it makes sense to share the task of being vigilant being awake between animals. Some animals will be asleep while others are awake so there's always a Sentinel who's on guard against potential threats. David Sampson and Charles Land didn't experiment a few years ago looking at the heads of people of Tanzanian hunter-gatherer people. They found that over the 20 days they looked at people's sleep at least one person was awake during 99.8% of the time were lapsed between the earliest person falling asleep and the latest person waking up. So there was pretty much always somebody awake during this sleep period keeping guard.