Human evolution; our 7-8 hour sleep pattern is not ape-like (Introduction)

by David Turell @, Thursday, January 24, 2019, 20:13 (431 days ago) @ David Turell

Parts of the world still have several periods of sleep, but most of the advanced world it is one long patch of seven-plus hours. No apes or other animals are like this:

"First off, we sleep less. While humans average seven hours, other primates range from just under nine hours (blue-eyed black lemurs) to 17 (owl monkeys). Chimps, our closest living evolutionary relatives, average about nine and a half hours. And although humans doze for less time, a greater proportion is rapid eye movement sleep (REM), the deepest phase, when vivid dreams unfold.


"While the costs of sleep are obvious — an animal is vulnerable to predators and other threats, and loses opportunities to find food and mates — the benefits are not. Different hypotheses about why we need sleep include neural development and upkeep, memory processing and immune defense, but there’s no consensus.

"Sleep habits also differ drastically among species. Elephants get by with two hours of shut-eye, while armadillos need 20. Researchers have found several factors that influence these variations in sleep patterns. For example, animals with high metabolisms sleep less — presumably because they spend more time awake and eating. And animals with bigger brains spend a greater portion of sleep in REM.


"In a 2018 study in the American Journal of Physical Anthropology, Samson and colleague Charles Nunn, an anthropologist at Duke University, employed a sophisticated statistical method to compare the sleep patterns found in 30 primate species, including our own. They found, says Samson, that humans are significant “evolutionary outliers.” We sleep less but spend about 10 percent more of our total sleep time in REM than expected. Human sleep is shorter and deeper — in other words, more efficient — than that of our closest relatives.

"The finding supports a hypothesis proposed by the duo in 2015: Efficient sleep gave our hominin ancestors an evolutionary edge. By shortening total duration, hominins reduced their time as unconscious targets for predators, and added waking hours to complete essential tasks, like learning, securing resources and maintaining social bonds.

"It’s also still unknown when our ancestors evolved this unusual sleep pattern. Samson speculates it may have emerged when they became too large to sleep in trees, roughly 2 million years ago with Homo erectus. While other apes avoid predators by building arboreal nests, it’s possible that hominins sleeping on the ground evolved more efficient sleep to allow them to spend more time awake — and on the alert for potential threats.

"Based on nearly 70 studies across cultures, including those without electricity or 9-to-5 workdays, Samson and Nunn determined that humans sleep an average of seven hours out of every 24. But, says Samson, “where it gets tricky is that when you look across cultures, the way those seven hours are expressed can be pretty flexible.”

"In contemporary industrialized societies, people typically sleep for one continuous bout. But other cultures divide sleep over multiple sessions, through daytime napping or two nighttime episodes, separated by about an hour of wakefulness.

The latter was the norm for humans before the Industrial Revolution, according to research by historian Roger Ekirch. In preindustrial documents, Ekirch identified over a thousand mentions of so-called first and second sleep, and activities done between, such as chores, prayers, even visiting neighbors. Found in newspapers, court records, diaries and literature, from Homer’s Odyssey to Leo Tolstoy’s War and Peace, the references permeate more than 2,000 years of recorded culture.

"The habit of segmented sleep was shed by the early 1900s, likely due to artificial lighting and changing societal views that equated single-bout sleep with productivity and prosperity. Yet Ekirch believes it persists, among Westerners who spontaneously wake in the middle of the night, “a persistent echo of a pattern of sleep ... dominant for literally thousands of years.”


"According to the sentinel hypothesis, staggered sleep evolved to ensure that there was always some portion of a group awake and able to detect threats.


" In a 2017 study in Proceedings of the Royal Society B, Samson gave activity trackers, which can approximate sleep time, to a community of Hadza hunter-gatherers in Tanzania. Over 20 nights, there were only 18 one-minute periods when everyone was asleep. Most of the time, about 40 percent of the group was active.

"The study “suggested there’s some type of mechanism set in place where there’s individuals alert, protecting everyone, while most everyone is asleep,” says Samson. This could explain how our ancestors avoided danger while enjoying deep, REM-packed sleep.

"It also fits the idea that the pattern emerged around 2 million years ago, when ancestors like H. erectus abandoned the safety of trees for a fully terrestrial life. Efficient, sentinel-style sleep may have then spurred advances in brain power, technology and social cooperation seen in later hominins. "

Comment: this is an other way we differ from all primates. Our brain seems to require it.

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