Why we sleep the way we do

Sleep_chimp_2

Who doesn’t love to sleep? The trouble is that some of us don’t do it very well, but perhaps that can be improved with a better understanding of sleep through evolution.

If you’re lucky, you’ll spend a third of your life asleep. “That’s pretty incredible if you think about it, because when we’re asleep we’re not doing things that are important for our survival or reproductive success,” says Charles Nunn, a professor of evolutionary anthropology at Duke University.

“We’re not searching for food, watching for predators, raising offspring, or searching for mates,” he told attendees at the International Society for Evolution, Medicine, and Public Health Inaugural Meeting on March 20, 2015 in Tempe, Ariz.

But nearly all animals show some signs of sleep. The birds do it, the bees do it, jellyfish, and fruit flies all sleep too. We know there are complex neurophysiological mechanisms going on during sleep. And we know that if we don’t get enough, our mental and physical health suffers, as is most clearly exemplified by the more than 70 million Americans who suffer from a sleep disorders.

When humans go to sleep, they first go into the lighter stages non-REM sleep, followed by the deeper stages (slow-wave sleep), then arouse out briefly, and go into REM sleep, where dreams come, arouse briefly again, and then the cycle continues. Each of these cycles lasts about 90 minutes to two hours, and is repeated, for around eight hours in humans.

But why has sleep evolved? More importantly, what is the evolutionary context of which human sleep has evolved? By researching these questions, Nunn says, it might just help us to better understand sleep disorders such as insomnia, narcolepsy, seasonal affective disorder, and circadian rhythm disorders.

The phylogeny of sleep

Nunn’s lab decided to create a database called “phylogeny of sleep,” for which they obtained data from 127 mammalian species, an average of nine individuals per species. They looked at total sleep duration, non-REM duration, REM duration, cycle length, and also whether or not sleep happened in one phase (mono-phasic) or if it was segmented (poly-phasic).

“What we find is incredible variation across mammals,” Nunn said. Among the expert sleepers included bats, while giraffes that spend very little time asleep (neck comfort might be an issue here).

The variation is explained by two categories of hypotheses: ecological, related to factors such as predation risk, and functional, related to brain size and function.

Although there is more evidence for ecological constraints than functional in regards to affecting sleep patterns, Nunn says.

The greater exposure to predation risk, the fewer hours spent in both non-REM and REM sleep. Herbivores are at greater risk for predation and sleep the least, for example.

The proposed functional hypothesis was that animals with larger brains needed more sleep, but Nunn’s lab found no significant association of this even when looking at regions of the brain like hippocampus.

“There were some differences, but nothing compelling,” he said.

Great apes, the nest-building primates

But there is one trait that unites all the great apes making them exclusive compared to that of all other primates in terms of sleep – “We build nests.” The great apes seek out bedding in which to lie down in, while monkeys and gibbons sleep in a more crouched position.

There’s some evidence that this nest building may be important for cognitive abilities too, based on experiments at the Indianapolis Zoo comparing sleep activities of great apes, postdoctoral associate David Samson.

Baboons had higher motor activity and sleep fragmentation, while orangutans sought out materials for better sleep comfort and had a deeper more efficient sleep, Samson found.

Compared to the great apes, humans have three notable differences, Nunn proposes:

Natural human sleep may be more flexible, we sleep less, and we have a higher percentage of REM sleep.

The flexibility of human sleep is observed among the Piraha hunter-gatherers of Brazil as noted by Daniel Everett who wrote that they take naps – 15 minutes to two hours – throughout the day and night. Because of these habits it can make it different for outsiders to sleep well among them.

Arizona State University anthropologist Ana Magdalena Hurtado noted that she also personally observed flexible sleep and staying up during the night among the Ache of Paraguay. “I’d hear activities going on all night long,” she said.

Historical evidence finds that humans often sleep in two phases, Nunn says. That suggests that humans are more polyphasic in their sleep, rather than monophasic, a change that might’ve occurred due to cultural changes. “Nowhere did people sleep from evening to dawn,” Nunn says.

Humans also tend to sleep only about seven or eight hours, the least of all primates, although they generally have a higher proportion of REM sleep compared with all the primates.

Knowing how human sleep patterns compare with the rest of the animal world, Nunn says we might help predict why we sleep the way we do. In the case of duration, we’re not an extreme outlier, but we’re on the left end (nearer to the herbivores). While in terms of REM percentage we are much more of an outlier, to the far right (nearer to omnivores and carnivores).

In the broader evolutionary context, the findings might inform treatment for key sleep disorders, but could also have greater public health implications. Example: “How do sleep patterns change with greater access to artificial lighting?”

Photo credit: Xavier Ortega

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