Thursday, July 12, 2012

On the evolution of sleep

We are not biologically programmed to rest a full night of consolidated sleep. In fact,
some societies favor shorter periods of sleep, during both the day and night.
Image via Creative Commons.

Assistant professor of anthropology at the University of California, Santa Cruz

Have humans evolved to sleep in a consolidated, nightly fashion, or is this some kind of social construct that we’ve fallen into?

There’s a nice write up on the evolution of diurnal behavior in humans by Cris Campbell, in which he uses my recent article in Current Anthropology to think about the relationships between economy, society, and sleep. I’m no hardline social constructionist by any means, but I’m sometimes concerned that evolutionary approaches to sleep can be fairly reductive. And one of the dangers of being biologically – and naturally – reductive is that we can come to accept things like American capitalism as the natural outgrowth of a particular pattern of human behavior (which I write about extensively in The Slumbering Masses). Some kind of middle road between biology and society is necessary to really see how sleep is shaped by social demands and how it impacts our biological well-being.

It sounds so reasonable, but it can come across as radical when I tell people that there’s no absolute human nature that determines our individual and collective actions, which is the basis of my argument in that Current Anthropology piece.

Rather than thinking of nature and nurture as absolute determinants of our behavior, it’s more appropriate to think of any individual behavior or social form as existing on a continuum between nature and nurture. That is, everything is somewhat natural and somewhat cultural (and sometimes, what we say is natural is actually cultural). Sleep is a great example of this: yes, we all have a natural, physiological urge to sleep, but how each person – and each society – organizes sleep varies, based on cultural norms and individual preferences. For some, this can mean nightly, consolidated sleep in an eight-hour chunk; for others, it might mean biphasic sleep – breaking sleep into two (or more) blocks of sleep, arranged throughout the 24-hour day. So our sleep styles might have developed out of evolutionary selection. Or it might be a little more complicated.

Biological anthropologists agree that niche construction can often interfere with (for better or worse) the process of evolution. Roughly, they mean that organisms of all sorts (including humans) can change their environments to maximize the possibility of their survival – think beavers building dams, which changes local ecology for the beavers as well as for the other animals, insects, and plants that are part of that environment. Humans, the usual argument goes, are niche constructors without parallel, having built complex societies, agricultural infrastructure, and cities. The assumption in much of the niche construction literature is that niches are positive – at least for the constructor. But humans may be able to build niches that are actually unhealthy for us. If humans had evolved to be biphasic sleepers, our pattern of consolidated activity throughout the day might be a very good example of a niche gone wrong.

The niche that Americans have built slowly, over the last 200 years, is one that consolidates our daily activities into one block in the day (say the 9-to-5 work schedule, alongside the 8-to-3 school schedule), followed by a period of recreation – usually taken up by dinner and nightly television – to be followed by our consolidated sleep. All of which begins again the following day. This kind of niche isn’t a byproduct of some inner nature, but rather a piecemeal construction that we’ve invested in over centuries of social development. And, if we look elsewhere, there are other models – including societies that favor biphasic or daytime sleep.

If we’ve developed a social structure based on our evolutionary desires for sleep, we could expect to generally not feel sleepy throughout the day and rarely see cases of insomnia. Because 30-40% of Americans claim to experience insomnia symptoms with some regularity, and because there’s a booming industry in alertness-promoting chemicals (Provigil, coffee, soda, tea, energy drinks, etc.), it would appear that our niche doesn’t really meet our needs. At its most benign, it might mean that we consume more caffeine than we should; but it might also be that the niche we’ve built is incredibly difficult for many to conform to, leading to experiences of sleep disorders.

There are at least two dangers in assuming our contemporary social structure is based on our evolutionary preferences. First, as I mentioned earlier, it naturalizes things like capitalism as inevitable outcomes of our selected-for behavior. Second, it means that disorderly sleepers aren’t just pathological and in need of treatment, but rather in need of evolutionary aberrations or throwbacks. That might sound silly, but similar ideas have been the basis for racism throughout history; as genomics provides a basis for our understandings of ourselves and others, we might also be facing a future of gene-based discrimination, not entirely different from Aldous Huxley’s Brave New World (which I also mention in that Current Anthropology piece).

Now, it may be that through this niche construction, we are slowly selecting against people who don’t sleep in accordance with it -- but with such a large, complex society, that’s unlikely to happen. One neurologist I know once said that our brains work best with a cup of coffee in our system. It’s a strange fantasy to imagine that we evolved over time through the selection of individuals who respond well to caffeine. Rather, it’s an accidental correlation between our physiologies and our lifestyles that leads us to really thrive on caffeine (for those of us who do).

It’s a lot safer to recognize that evolution isn’t purposeful in all of its selections; some selections are accidents, although they can be beneficial. What we can select are the social models that govern our lives, and other models are possible, as organizations like the Take Back Your Time movement have advocated for. And what we should be working toward are social forms that meet the needs of all sleepers, not some or even most. Recognizing that society can be different – and more flexible – also accepts that variation within the human species is non-pathological, and that there might be better ways to think about difference than as disorderly.


Matthew Wolf-Meyer is assistant professor of anthropology at the University of California, Santa Cruz, and author of The Slumbering Masses: Sleep, Medicine, and Modern American Life, available in October 2012 from University of Minnesota Press. He blogs regularly here.

"A groundbreaking contribution to our understanding of sleep and its manifold discontents. With scrupulous care, Matthew Wolf-Meyer probes the current state of sleep medicine as well as its absorbing history. At a time when modern society’s dependence on sleeping pills and plush bedding has never been greater, The Slumbering Masses is all the more welcome for its ambitious compass and penetrating insights."
—A. Roger Ekirch, author of At Day’s Close: Night in Times Past 

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