Day In, Day Out

Intersections of biology and everyday life

Insomnia? Or Evolution?

Should variations in human sleep be targeted for medical interventions?

Something woke you up in the middle of the night. The tug of the need to urinate? A bedpartner’s jerky limb? A loud noise? A startling dream? Whatever it was, the event passes as you bring yourself to unsteady consciousness. You lay in the dark for a few minutes — for what seems like a few minutes — deciding whether or not you’re going to get out of bed, if even to go to the bathroom quickly. After another minute of lying in the dark, your bladder has convinced you to go to the bathroom — maybe then you’ll be able to get back to sleep. But once you’re in the bathroom, you know it’s all over. You’re awake. You hadn’t even turned on the lights for fear that doing so would make returning to sleep impossible, but as you fumble in the dark, you know that night has come to an end and your day is starting very early.

The experience is generally referred to as sleep maintenance insomnia. It is characterized by being able to fall asleep when one wants to, but awakening in the middle of the night and being unable to get back to sleep. According to the National Sleep Foundation and drug manufacturers, millions of Americans experience sleep maintenance insomnia on a regular basis. From the perspective of modern science and medicine — and society more generally — this is disorderly sleep. If you wake up after four hours and stay up until the following night, you aren’t getting the amount of sleep you need in order to get through the day. Yet from the perspective of history, being unable to get back to sleep immediately might have everything to do with human evolution.

Humans may have evolved to sleep in a biphasic or non-consolidated fashion, that is, we may be physiologically inclined to sleep in two or more periods over the 24-hour day. We have unambiguous evidence that in pre-industrial Britain and the United States — so before 1840 — that people slept in two periods at night. They would lay down to sleep around sunset or shortly thereafter, wake up around four hours later for a couple of hours, and then sleep again for a few more hours. Today, despite pressures to stop doing so from some quarters, napping cultures thrive in southern Europe, China, Taiwan and elsewhere — people sleep for several hours at night and supplement this sleep with a hefty nap during the day, upwards of two hours.

Sleep is comprised of a series of cycles, which last about two hours for most people. During each cycle, we move through non-Rapid Eye Movement and Rapid Eye Movement (REM) sleep. At the end of each cycle, we move towards wakefulness, and this is when people often wake up. When we wake up in the middle of a cycle — due to an alarm clock or emergency — we often feel terrible throughout the day, struggling with an unresolved sleep cycle. (Incidentally, there are now alarm clocks that detect your progression through a sleep cycle and wake you up at just the right time.) When we think about this from the perspective of evolution, waking up every couple of hours to check your environment is a pretty useful adaptation — sleeping deeply through the night puts one at risk of nocturnal predators. But modern society favors consolidated sleep, so those of us who still sleep as our ancestors did are at risk of being diagnosed with sleep maintenance insomnia.

There aren’t any drawbacks to sleeping in a less consolidated fashion. Some evidence suggests that the grogginess we experience upon awakening is lessened and that we wake up more easily when we sleep for shorter periods. But society is structured around consolidated sleep — as I discuss in The Slumbering Masses, very few employers offer onsite napping facilities — and spending 12 to 14 hours in bed each night would cut into work and family time. And so, even though biphasic sleep might work for us physiologically, it might not work so well socially.

This is why sleep maintenance insomnia is treated as a sleep disorder and not normal human variation: it’s disruptive to society. It can be a nuisance to individuals as well — being chronically sleep-deprived can lead to serious social and health problems — but it wouldn’t be such a nuisance to individuals if society was set up to allow for people to sleep the ways they want to. American sleep patterns are more indebted to our ideas about the workday and school day than any basis in human nature or evolution. Some sleep disorders are serious and benefit from medical attention. But people who experience sleep maintenance insomnia might benefit more from a midday nap than a pharmaceutical fix or a large coffee. It’s up to us all to think about how society might better reflect our needs for sleep — to invent social arrangements that benefit us rather than pharmaceutical companies and the corner Starbucks.

Matthew J. Wolf-Meyer, Ph.D., is a cultural anthropologist and historian of science and medicine at the University of California, Santa Cruz.

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