Science Of...

There's An Evolutionary Reason Older People Get Up Early

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When it comes to age and sleeping habits, there are a lot of stereotypes. The elderly show up at the mall the minute it opens, eat dinner at 4:30, and turn in for the night before the sun even sets. Young people are out causing mischief until the wee hours and lazily sleep in 'til noon. Why is there such a sleep mismatch between the old and young? A study suggests that this bedtime quirk may have helped our ancestors survive. It's not a bug; it's a feature.

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To learn more about how early humans may have lived, scientists often turn to modern hunter-gatherer societies. Although they live in the modern world, some elements of their lifestyle have endured through the centuries. That means life in hunter-gatherer societies is often pretty similar to how researchers think our human ancestors once lived, before the advent of agriculture.

For a study published in Proceedings of the Royal Society B, researchers recruited volunteers from the Hadza, a hunter-gatherer society in northern Tanzania. They live in groups of 20 to 30 people, spending their days finding berries, tubers, and meat in the savanna woodlands and spending their nights sleeping outside next to the fire or indoors within woven-grass huts. Importantly, they sleep on the ground without artificial lighting or climate control, making their environment close to that of early humans.

The researchers had 33 men and women between the ages of 20 and 60 to wear a sleep-tracking watch for 20 days. They found that very few of the Hadza slept at the same time. Some people, generally older participants in their 50s and 60s, went to bed as early as 8 p.m. and woke up at 6 a.m. 20- and 30-somethings, meanwhile, often stayed up until past 11 and slept in after 8. Most participants got up a few times in the middle of the night to tend to crying babies or relieve themselves. Been there.

Insomnia Like A Fox

The fact that older Hadza people go to bed earlier might not be that surprising — your grandparents probably do too. But here's the twist: out of more than 220 hours of observation, there were only 18 minutes in the entire study where all 33 participants were asleep at the same time. On average, more than a third of the group was awake (or at least only lightly resting) at any given time.

Researchers think that this sleeping mismatch helped our ancestors survive. Predators can attack at any hour, and if a few people are always awake, someone is always around to guard against danger. Plus, you don't need to assign people to keep watch overnight if someone's already going to be up. Bottom line? If you're getting older and having trouble sleeping past 7 a.m., there may not be anything wrong. It's probably just millennia of evolution helping you watch out for lions.

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Key Facts In This Video

  1. In a lifetime, a person will spend 24 years asleep. 00:13

  2. After a few nights of sleep deprivation, the body can fully recover with a few nights of good sleep. 01:04

  3. The ideal average length of sleep is seven to eight hours, but it may vary from person to person. 02:31

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