Where Human Sleep Went Wrong

When evolutionary anthropologist David Samson was living and working among the Hadza tribe in northern Tanzania, he noticed something puzzling: Their sleep was highly fragmented, short in duration, and low in “efficiency,” or actual time spent sleeping versus time in bed. This broken sleep was partly the consequence of activity and noise well into the night in the camps. People stayed up late telling stories, sharing food, and dancing. Yet, the Hadza uniformly reported high satisfaction with their sleep. It challenged the so-called Paleo sleep hypothesis, the notion that hunter-gatherer sleep must be optimally long and deep, but also ran counter to the medical orthodoxy that unbroken sleep is essential to a good night’s rest.Featured VideoSamson traveled to Tanzania to hang out with the Hadza because he was trying to untangle what he calls the sleep paradox: Sleep is critical to human functioning, and yet, we sleep fewer total hours than any other ape, and are still arguably the most evolutionarily successful of the primates. This riddle sent him climbing into chimpanzee nests high in the trees and exploring the sleeping huts and practices of communities around Africa and Madagascar.The stories he collected, people he met, and research findings he uncovered are vividly described in his new book The Sleepless Ape: The Story of Sleep in Human Evolution. I spoke with Samson about what we really need for a good night’s sleep, why we may be on the cusp of a “sleep enlightenment,” and the origins of what he calls the “lie-down-and-die” model of Western sleep.What is the paradox of human sleep?You can just say, “Oh, here’s where humans are relative to other primates on sleep.” But that misses the deeper evolutionary story. When I was a postdoc at Duke University about a decade ago, new data was emerging showing that even after controlling for brain size, body size, social order, and actual phylogenetic relatedness, humans are weird outliers—we’re the shortest sleeping primates, yet we pack in the most REM sleep relative to this short duration.This emerged to me as one of these mysteries that I had to sink my teeth into, and it eventually became the thesis of the book. As humans, we all know what it’s like to have a bad night’s sleep. If you have only two or three hours under your belt, your cognition suffers, your capacity to plan ahead suffers, your social regulation suffers. REM sleep functions almost like a nightly behavioral therapist. Sleep deprivation also erodes your ability to resist disease and to resist injury. We know experientially on a day-to-day basis how desperately we need sleep, yet we’re the shortest sleeping primate ever recorded on the planet, by a very far measure. The owl monkey, by contrast, sleeps 17 hours in a 24-hour period. That’s the paradox in a nutshell.Read more: “What We Can Learn from an Insomniac Fish”Part of why we’re so different from other apes is that we descended from the trees around 1.8 million years ago and learned to sleep in tight formations on the ground, as you note in the book. What else was going on at that time that allowed this shift to happen?I use the acronym SHELL to explain the shift.S is for shelter. You can build a sleeping platform if you’re a great ape, or a bird-like nest if you’re a galago or a lemur. But what we were doing by 1.8 million years ago was building physical micro habitats out of, say, acacia, like the Hadza. That narrows the variations in the extremes of either cold or hot temperatures.H is for heat. Mastery of fire, if you’re a proponent of Richard Wrangham’s cooking hypothesis, gave us a thermal regulatory buffer.E is environmental preparation. We lived in camps and bands of adults that moved seasonally, much like foragers do, something you can see in the paleoanthropological record. Every time we moved, even generationally, we enhanced these environmental spaces, making it a little bit better for next season.L number one is for lux, or light. Constant exposure to natural ambient light allowed our circadian rhythms to sync to real-time sensory signals around us.L number two is for lookouts. Because we were living in that social group, we also had socially buffered security for our sleep sites.Together, this created a novel sleep ecology, what I call a physical sleep exophenotype. An exophenotype is when a genetically driven instinct reshapes the environment, which in turn shapes a species behavior, feeding back into its evolution. A classic example is a beaver dam. Beavers create these fantastic feats of engineering, creating ponds artificially that then push them to specialize foraging skills suited to that artificial environment. That’s the kind of feedback loop that we got into one and a half million years ago that led to the drastic evolutionary changes in our sleep relative to other primates.How did these shorter but more REM-saturated patterns of sleep influence human cognition? You mention one 2009 study in the book, “The Role of Sleep in Cognition and Emotion,” which was influential in your thinking about this.Yes, that’s a brilliant paper by Matthew Walker, who makes the argument that REM sleep is a nighttime therapist. During REM sleep, you strip the emotional charge off a memory, so that when you recall it later while awake, you don’t have to re-experience the original emotion. It’s the science behind, “Sleep on it, honey, you’ll feel better in the morning.” You release the affective energy so that you can process the information without the emotional weight attached.That’s why one of the leading edge therapies for PTSD now targets sleep quality directly. Trauma disrupts sleep, which prevents that affective release. Treat the sleep and the nightly passive therapy can resume, helping to resolve trauma symptoms.If sleep is so important to so many functions, why is insomnia so common in humans?First, I’m not a clinician. Always consult your physician before you listen to me. I’m just an evolutionary anthropologist. But my intuition is that we over-problematize our sleep. One of the framings from evolutionary medicine involves hypervigilance. Let’s say somebody comes in with insomnia, you can reframe it as, “Hypervigilance was probably quite adaptive in an ancestral condition.” What you have is a perfectly normal outcropping of a psychological adaptation, not something broken. You’re hypervigilant at night, which probably served many of your ancestors very well. You wouldn’t be here if not for some level of hypervigilance. Reframing it that way alone tends to reduce symptoms.The problem arises when your body can’t distinguish, on a mechanistic level, between your 9 a.m. PowerPoint to the board and a tiger stalking you in the bush. The HPA axis—the hypothalamic-pituitary-adrenal axis—cannot distinguish between the two. It burns hot either way, and that can produce insomnia.The other angle is an evolutionary mismatch in our circadian physiology. We evolved for how things were, not how things are. Today, we spend way too much time indoors. We’re sedentary. Right now, I’m in a temperature-controlled room, and it’s hot outside. My body’s being denied crucial information about temperature oscillation. That information normally helps a dozen or so independent clocks in my body coordinate physiological processes. Light and the temperature cues are crucial, and we’ve been disrupting them ever since Edison’s light bulb. Maybe even earlier, since the invention of fire, although fire’s much less a culprit because it doesn’t emit blue-wave light. Blue-wave light is the real problem because it inhibits melatonin production.You say we evolved for how things were, not how things are, so to get a picture of how things were, you studied the sleep customs of a number of hunter-gather tribes, like the Hadza and Himba, and Malagasy communities in Madagascar. What does a good night’s sleep look like for them?Well, there’s a lot of variation by ecology and socio-technological adaptation. Not all small-scale societies are alike, but there are patterns. One paper we just published when I was writing the book found that people in large-scale societies were sleeping longer, and with higher sleep quality than people in small-scale off-grid societies. Sleep efficiency—time spent asleep versus time spent in bed—averages around 88 percent in the global north versus 74 percent in small scale off-grid societies. Sleep scientists are usually thrilled to improve someone’s sleep by 10 or 15 minutes a night. These are categorically larger differences.It’s kind of counterintuitive, right? It flies in the face of the sleep epidemic hypothesis. For the past 15 years, the CDC has painted modern global northern sleepers as the worst sleeping cohort of all time, like we’re in a sleep dystopia. I never bought it, because I’d just started doing field work in small-scale foraging societies. I’m like, “No way. They’re partying all night.” But they can, because there’s no top-down pressure to go into the office at 8 a.m. They’re free to catch supplemental sleep as needed throughout the 24-hour period.Most importantly, when we measured their circadian function directly, small-scale communities had much higher amplitude circadian rhythms than global north populations, which I think is why they report being happy with their sleep even though it’s more fragmented. That satisfaction with their sleep seems to be underpinned by healthy, strong circadian function.How do you square the fact that these small-scale societies are so happy with their fragmented sleep with the clinical evidence that links short, broken sleep to so many different disorders and health problems, such as diabetes, obesity, Alzheimer’s, heart disease, cancer, and infertility?The science is catching up. Sleep and chronobiology, the study of circadian rhythms, have historically been completely separate fields. Now, they sit side-by-side at conferences, because they’re deeply interlinked. Looking at sleep duration in