Something woke you up in the middle of the night. A partner’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—at least, what seems like a few minutes—deciding whether you'll get out of bed, if even just to go to the bathroom. Finally convinced by your bladder to do just that, you hope that maybe you’ll be able to get back to sleep afterward. But once your feet hit the floor, you know it’s all over.
And even as you fumble in the dark, you know that your night has come to an end and your day has started very early.
This type of experience is generally referred to as sleep maintenance insomnia. It is characterized by being able to fall asleep when one wants to, but then awakening in the middle of the night and being unable to go 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 in general—this is disorderly sleep. If you wake up after four hours and stay up until the next night, you aren’t getting the sleep you need to get through the day.
Yet being unable to get back to sleep immediately might have everything to do with humen evolution.
We 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 course of the day. We have unambiguous evidence that in pre-industrial Britain and the United States—roughly, before 1840—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 economic pressures against it, napping cultures still thrive in southern Europe, China, Taiwan, and elsewhere—people sleep for several hours at night and supplement this sleep with a hefty nap, upwards of two hours, during the day.
Our night's sleep is comprised of a series of cycles, each of which lasts about two hours for most people. During each cycle, we move through Rapid Eye Movement (REM) and non-Rapid Eye Movement sleep. At the end of each cycle, we move toward wakefulness—and this is when people often wake up. When we wake up in the middle of a cycle, due to our alarm clock or some emergency, we often feel terrible throughout the day, struggling with an unresolved sleep cycle. (There are now alarm clocks available that can detect your progression through a sleep cycle and wake you up at just the right time.) When considered from an evolutionary perspective, waking up every couple of hours to check your environment is a pretty useful adaptation—sleeping deeply through the night puts one at risk from nocturnal predators. Modern society, though, favors consolidated sleep, so those of us who still sleep as our ancestors did may be diagnosed with sleep maintenance insomnia.
But there aren’t any drawbacks to sleeping in a less consolidated fashion. Some evidence suggests that the grogginess we typically experience when we wake is lessened, and that we wake up more easily when we sleep for shorter periods. But our 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 does not work as well socially or professionally.
This is why sleep maintenance insomnia is treated as a sleep disorder and not regarded as a 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 for individuals if society would allow people to sleep the ways they want to. American sleep patterns owe more to our ideas about the workday and school day than any research into 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 fashion social arrangements that benefit us rather than our institutions.