Any behavioral advantage in dealing with highly dangerous events would have increased the probability of reproductive success. A dream-production mechanism that tends to select threatening waking events and simulate them over and over again in various combinations would have been valuable for the development and maintenance of threat-avoidance skills. Empirical evidence from normative dream content, children’s dreams, recurrent dreams, nightmares, post-traumatic dreams, and the dreams of hunter-gatherers indicates that our dream-production mechanisms are in fact specialized in the simulation of threatening events, and thus provides support to the threat simulation hypothesis of the function of dreaming.
Carol Worthman, meanwhile, believes that the presence of predators also formed our habits of sleeping, which steers us in another direction, toward the flip side of fear. If we look closely, we can find ample evidence that sleep is not retreat but an act of social engagement, and this, too, can be derived from wondering where the lions are.
Worthman is an anthropologist and probably the only one in captivity who specializes in sleep. We caught up with her at Emory University; she’s a polite and engaging woman in a neat and fully organized office. She had just returned from a stint in India teaching the scientific method to Buddhist nuns and a stop in Vietnam to check on research in a remote village where television had been introduced for the first time.
Years ago, Worthman tackled the topic of sleep, mostly out of curiosity and her wonder as to why there was not more on the subject in the anthropological literature, given the importance of sleep in our lives. She did a survey of cross-cultural research on sleep habits and found, much as Stickgold found in research on the reasons for sleep, that we know almost nothing. There is some excuse for this. People sleep in private, or at least we once assumed so. Further, unlike spear tips and hand axes in hunting and fire smudges in cooking, sleep leaves no trace in the archaeological record, not much in stories, and not much in our bones—and so there is precious little to go on if we need to ask the very question about sleep that we have been asking in this book about other endeavors, like food and movement: what is the evolutionary history of this fundamental human activity?
It turns out that the anthropological perspective does nothing to contradict Stickgold’s conclusions from studies of modern-day sleepers, but it does provide a different emphasis, especially on the prescription. There is nothing in the cross-cultural studies that disagrees with the idea of the need for a baseline of sleep of about eight hours out of every twenty-four, but Worthman says her real concern is with quality, not quantity. For instance, she says people who complain about insomnia—the torturous variety that has one lying in bed awake, tossing and turning through the night—often sleep far more than they report, but they get only low-quality sleep. They believe they are awake—and, more to the point, the sleep they do get doesn’t do them a lot of good.
“The question is how do you get good sleep, and that draws attention to context, and that’s where the evolutionary context can be helpful,” Worthman says.
What we know about evolutionary context is extrapolation from what we know from cross-cultural sleeping habits. But nonetheless, the studies contain some clear evidence that we are missing something important about context, at least we who practice what Worthman calls the “lie down and die” model of sleep: to bed at ten, lights out, silence, set the alarm, and await resurrection. The simple fact is, across the world and across time as far as we know, few cultures sleep this way.
“In virtually all societies there is a sense of the social organization of sleep, and in many, many societies the provision of an appropriate sleep context is viewed as extremely powerful,” she says.
And what does an appropriate context look like? To begin with, it includes other people. Few other cultures view sleep as a retreat, even a private act. Just the opposite.
Back to the lions for a moment, to see where that comes from. Thomas described a scene as much about being awake as it was about being asleep, which makes perfect sense if you happen to be a !Kung sleeping outside among lions, and through most of evolutionary time, humans did indeed sleep outdoors among predators. But there is actually some math at the root of this casual observation, calculations that Worthman has done. This is based on well-known and established variations in sleep patterns that remain fixed in modern humans, according to age. Babies can be and often are awake at seemingly random periods around the clock, but once they are a bit older, they lock onto a circadian rhythm much like that of adults. Adolescents, however, have a rhythm of their own, worldwide and across cultures: they go to bed late and get up late, compared with adults. Older people, meanwhile, are often awake longer and for periods in the night. This age segregation is consistent across cultures but begins to make sense when one superimposes those various patterns on one another. Worthman says that doing so allows a calculation that, given a band size of about thirty-five people with usual age distributions, yields a group pattern in which at least one person is awake at any given time.
Yet there is more to this than simply being awake. Many cultures, for instance, cultivate a form of light sleep, a watchful doze that is instantly reversible. In the studies of modern sleepers, this corresponds with a stage of sleep that confers a distinct set of benefits. Everyone performs this sort of light sleep without realizing it, but each of us also needs periods of very deep sleep, a stage vital to brain benefits and at the same time deeply threatening to people who live among lions. Researchers generally divide sleep into two categories by eye movement: rapid eye movement, or REM, sleep and non-REM sleep. The latter category has four distinct stages. Both REM sleep and the deepest stage of non-REM sleep are marked by a near complete lack of muscle tone and awareness; it’s like being in a coma, helpless against threats like predators.
In REM sleep, debilitation is nearly complete. Two pathways of brain chemistry work together to induce paralysis in all muscles but the eyes. Researchers don’t know the function of this paralysis but speculate that it prevents injury caused by muscles acting out our dreams, which also mostly occur in the REM state. People with disorders that prevent this paralysis often suffer such injuries.
Worthman says these stages, more than even the threat of lions, are why the social context of sleep is vitally important. We are not checked out or mentally absent during sleep, at least not all the time. On the contrary, as the research has shown, our brains are doing some of their heaviest lifting during sleep. But doing that requires modulation, a shifting of gears from one stage of sleep to another, which in turn requires some attention to context, reading of the signals that tell us when it is safe to check out and become helpless against external threats. In order to sleep properly, we need to pay attention to what is going on around us, using that awareness to guide us through the necessary stages of sleep. Isolating ourselves in soundproof rooms may be about the worst way there is to go about this—but, more to the point, so is isolating ourselves from other people.
This conclusion is not speculation. Worthman carried out one research project in Egypt, which gave her access to subjects who have been settled in cities for millennia. The choice was deliberate: she wanted to look at the persistence of hunter-gatherer sleep patterns, despite civilization. Egyptians, in both city and countryside, sleep the way most of the world does, which is to say together—what she calls “consolidated sleeping.” Typically, whole extended families sleep in great rooms, with almost no isolation. There are exceptions, though, and those proved to be the most telling. Egyptians and others typically segregate postpubescent girls from boys. Not always. Sometimes there is an aunt or grandmother and the teenage girl bunks with her. But some end up sleeping alone, and it was those people, both boys and girls, who had the insomnia and other forms of dysregulation. The people who slept alone had the emotional problems.
This same pattern has emerged in a variety of studies to the point that we begin to understand why social sleeping seems to be a nearly universal characteristic of cu
ltures, as are the staggered patterns of wakefulness of the group. While we are sleeping, we continue to monitor our surroundings for cues of safety: relaxed conversation, relaxed movement of others, popping fire. Those cues, subtle sounds signaling safety, tell us we can retreat to our deepest sleep.
Many cultures are, in fact, conscious of all of this and the importance of these arrangements, and no place is the importance more pronounced than in the case of infants. (We need not necessarily bring lions into the picture to underscore this, although predators like hyenas and leopards are certainly preferential to the young of our species, undoubtedly one of the reasons infant mortality was high among our ancestors. Thomas, for instance, records one example of great injury to a toddler who stumbled into a campfire while others were sleeping, and this was probably a common occurrence through time.) One of the biggest reasons for modulated sleep is to protect infants.
But the research suggests that this works in both directions—that is, infants’ bodies are instinctively aware of their vulnerability and so do things like dream about frightening animals. They are even more dependent than adults on signals of safety. All of this helps explain what Worthman characterizes as an almost universal perplexed response among most other cultures upon hearing of the Western practice of making babies sleep alone.
“They think of this as child abuse. They literally do,” she says.
The evolutionary context of sleep, however, extends well beyond the people around us, and this may begin to suggest some antidotes to our present isolation, some practical steps one might take to reintroduce evolutionary context to our rest. Anthropological studies have shown that almost all cultures pay a great deal of attention to the sound of a fire, and not just as a threat to babies. Changes in the crackle and pop might, for instance, signal that a fire is dying and trigger a new level of alert sleeping, just as the sounds of a fire settling into a sustained glow might signal that it’s okay to sleep deeply. This doesn’t mean you need to sleep next to a fire (although it’s nice if you can). But you can look for similar patterns of sound that may help, even recordings.
Likewise with animals. Herders in particular sleep with such sounds as cud chewing and gentle bovine breathing, signals from sentinel animals of peace that transpires when no predators are around. Our favorite sentinels through evolutionary time were once predators: wolves slowly tamed by food to be dogs. Any suburban dweller can attest how the sound of an incessantly barking dog can be profoundly disturbing to sleep, more than decibels and persistence alone can account for. Yet we forget how the reverse is certainly true: that many of us tune our degree of peace and relaxation to the rhythms of a snoring dog. If something were wrong, the dog would say so.
All of this may be enough to explain the finding of epidemiology that people who are married and people who have pets live longer. It may be because they sleep better.
AGAIN, VARIABILITY
In one of her papers, Worthman wrote something that circles this whole idea back to fat, sick, and stupid. Like Stickgold, Worthman is deeply impressed with the homeostatic nature of sleep, that the body has an overwhelming sense of needing it and, left to its own devices, will do what must be done to ensure adequate sleep.
It’s a wonderfully fluid phenomenon, she says, which is why most cultures don’t worry about sleep or even losing sleep. If you’re awake tending the fire or sheep one night, no big deal: doze through the afternoon and catch up.
She’s seen conversations like a business negotiation among, say, a group of men in Egypt that conclude when one guy simply falls asleep.
“Literally, a guy will just pull a cloth over his head and go to sleep,” she says. “And that’s no biggie. It’s not like a guy just slid under his chair in the boardroom. It’s not asocial. If you view sleep as a social behavior, then it is just integrated in life.”
But this is only true if our bodies are left to their own devices, and our rushing, wired world has all sorts of devious mechanisms for overriding those controls. That’s the dilemma. How do we get away with this, this deficit that accrues not just for a couple of days but day in and day out, through whole careers? Where is homeostasis? How do our bodies balance this behavior?
Worthman’s answer is this: we pay for sleep deficit in the currency of stress.
“Sleep deprivation looks like stress. It increases cortisol, it increases appetite, decreases satiety, increases blood glucose levels,” she says. “This is straight out of the stress literature. If you curtail sleep just now and then, you can manage the hit, but if you do it too much, it erodes the health of the organism, the person, and her ability to cope.”
This, too, is a case for modulation and adaption. On the one hand, sleep is far more elastic than we make it out to be. Indeed, it is wrong to characterize it as inactive or in retreat. Rather, it is a dynamic state important to brain function and some of our most important work. More to the point, sleep is when we do some of our most important work, both in processing information and, as the cultural studies show, in engaging others and building social bonds of trust. And because it is so important, it is adaptable and fluid. That is, our bodies are hardwired with a series of circuits to allow sleep to flow with the needs and demands of our day, what Worthman means when she calls it “fluid.” And as with most other cases of our adaptability, we need to practice adapting to strengthen that skill, to modulate, to read the signals and cues that attach us to our physical and social environment, flexing the adaptive tools like muscles.
This is just like the response to stress that makes us stronger in exercise, that is unless that stress becomes chronic and unremitting, day in and day out, and then we pay in the currency of cortisol and inflammation: fat, sick, and stupid. Complete recovery of the evolutionary context of sleeping is probably lost to us, but we have the basics. We probably don’t know enough and have lost the proper environments to make sleep perfect, but at least we have some clues as to how to make it better. That and ample evidence that we ought to do so.
For many of us, this is a simple matter of ensuring sufficient sleep, the fundamental issue in our overstimulated, overcaffeinated lives. It is true that much about sleep and sleep disorders remains a mystery, especially when we begin considering the evolutionary rules, as we have here. But one important fact we know for sure is that you must get sufficient sleep, and way too many of us don’t.
Beyond this, evolution provides some hints about the proper context of sleep: Irregularity is okay. So are naps. A sense of safety is critical. If you can, sleep around others, and this may include traditional sentry animals like dogs. Some people have even found that the relaxed sounds of conversation typical of all-night radio do the trick. Avoid alarming sounds like sirens. Look for safe sounds like the lapping of gentle waves (a signal of safe weather, no storms) or a settling wood fire. Try recordings if you can’t be near the real thing.
And then there is light, and in this there is an intriguing clue that suggests we have so much to learn. Much has been made of this issue of light as it affects our sleep, and the advent of artificial light is right up there with agriculture as one of the more profound shifts in the conditions of our existence, particularly for those humans who ventured north and south to where day’s and night’s lengths vary widely.
Of course, we can argue that artificial light has been with us a very long time; we’ve already talked about the importance of fire. The famous cave painters at places like Lascaux and Chauvet had fat-fueled lamps that they used to execute their artwork as long as forty thousand years ago. But fire and fat produce light of very different wavelengths than electric lights do and, more to the point, much dimmer light. This is key. The real problem is light bright enough to mimic the sun. Virtually all living organisms, even plants, are finely tuned to cycles of light and dark, to the passing of days, but also of seasons, patterns of life called circadian rhythms. Evolution has embedded layers on layers of mechanisms in humans to honor these cycles, and there has been plenty of research into at
least one of these layers to tell us light is pivotal, not just to sleep but to our health and longevity in general.
The mechanism is pretty simple. Sunlight strikes a tiny gland behind the eye called the pineal gland, which in turn regulates the production of melatonin, the hormone that governs sleep and our circadian rhythms. Any artificial light that approaches the brightness of the sun is enough to trigger this same process, and the research shows that the everyday, garden-variety 100-watt lightbulb is enough. The average office is about three times as bright as the threshold. The effect ratchets up with certain wavelengths, especially those that produce blue light, which takes us to electronic devices and televisions, all of which mess with melatonin. The blue wavelengths are accented in these devices, as they are in any light-emitting diodes (LEDs), such as those in super-energy-efficient lightbulbs. Research has already demonstrated a clear effect of computer monitors on melatonin, largely because of the specific wavelengths emitted.
Yet the effect of even the simple lightbulb goes a lot farther. Electric light conquered the night and made things like shift work possible, allowing people to work around the clock. Even those not working are more active than our ancestors, creating cities that never sleep and a regimen of noise that is unavoidable and takes a toll on our rest. Thus it gets hard to sort out the root cause of the damage—noise or light—but also the answer to this question: is the damage caused by lack of sleep or by disruption of the powerful circadian rhythm?
Nonetheless, the damage is there, and the studies bear this out. Nurses who work night shifts, for instance, are more likely to develop breast cancer. That same group had a 35 percent greater chance of suffering colon cancer. Studies have linked the disruptive effects of artificial light at night to depression, cardiovascular diseases, diabetes, and obesity. We believe that light is an enormous factor in problems like attention deficit disorder. And while it may be difficult to control the social conditions of sleeping for some people in some situations, most of us have much more control over light. An effective intervention here might be as simple as dimming all the lights and shutting down the television and computer monitor a few hours before bedtime. Another might be as drastic as finding a different job if the current one makes you work at night. But when weighed against issues like an elevated risk of colon cancer, such measures seem less drastic.
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