More than one-third of American adults wake up in the middle of the night on a regular basis. Of those who experience "nocturnal awakenings," nearly half are unable to fall back asleep right away. Doctors frequently diagnose this condition as a sleep disorder called "middle-of-the-night insomnia," and prescribe medication to treat it.

Mounting evidence suggests, however, that nocturnal awakenings aren't abnormal at all; they are the natural rhythm that your body gravitates toward. According to historians and psychiatrists alike, it is the compressed, continuous eight-hour sleep routine to which everyone aspires today that is unprecedented in human history. We've been sleeping all wrong lately — so if you have "insomnia," you may actually be doing things right.

The flip of a light switch

"The dominant pattern of sleep, arguably since time immemorial, was biphasic," Roger Ekirch, a sleep historian at Virginia Tech University and author of "At Day's Close: Night in Times Past" (Norton 2005), told Life's Little Mysteries, a sister site to LiveScience. "Humans slept in two four-hour blocks, which were separated by a period of wakefulness in the middle of the night lasting an hour or more. During this time some might stay in bed, pray, think about their dreams, or talk with their spouses. Others might get up and do tasks or even visit neighbors before going back to sleep."

References to "first sleep" or "deep sleep" and "second sleep" or "morning sleep" abound in legal depositions, literature and other archival documents from pre-Industrial European times. Gradually, though, during the 19th century, "language changed and references to segmented sleep fell away," said Ekirch. "Now people call it insomnia."

You can blame the shift in your sleeping habits on Thomas Edison's lightbulb and the Industrial Revolution.

Ekirch explained that in the past, and especially during winter, darkness spanned up to 14 hours each night. Except for those affluent enough to burn candles for hours, folks were left with little to do but go to bed early, and this gave a great deal of flexibility to their nightly sleep requirements. Segmented or biphasic sleep patterns evolved to fill the long stretch of nighttime, and as observed by anthropologists, segmented sleep continues to be the norm for many people in undeveloped parts of the world, such as the Tiv group in Central Nigeria.

In places with electricity, though, artificial lighting has prolonged our experience of daylight, allowing us to be productive for longer. At the same time, it has cut nighttime short, and so to get enough sleep we now have to do it all in one go. Now, "normal" sleep requires forgoing the periods of wakefulness that used to break up the night; we simply don't have time for a midnight chat with the neighbor any longer. "But people with particularly strong circadian rhythms continue to [wake up in the night]," said Ekirch.

In the 1990s, a sleep scientist named Thomas Wehr discovered that everyone sleeps biphasically when subjected to natural patterns of light and dark. In Wehr's well-known study, he subjected participants to 14 hours of darkness per night, and found that they gradually shifted to a routine of taking two hours to fall asleep, then sleeping in two four-hour phases separated by about an hour of wakefulness—a pattern that exactly matched Ekirch's historical findings.

[Infographic: The data from Wehr's sleep study]

Wehr concluded that biphasic sleeping is the most natural sleep pattern, and is actually beneficial, rather than a form of insomnia. He also inferred that modern humans are chronically sleep-deprived, which may be why we usually take only 15 minutes to fall asleep, and why we try our best not to wake up in the night.

One benefit of biphasic sleeping may be that it makes it easier to recall and access dreams. Wehr's study subjects normally awakened from REM sleep, which is the deep sleep stage during which dreams occur. According to Ekirch, the historical evidence bears that out. "Waking up directly after dreaming afforded people a pathway to their subconscious," he said. "With morning dreams we don't have the opportunity to let our dreams settle. The light goes on and we get out of bed immediately. So in short, we have lost what people in the past regarded as a critically important part of their lives – their dream life."

Sleepers set in their ways

Wehr's and Ekirch's results are becoming more and more widely known, and psychiatrists and sleep specialists are beginning to implement them. However, the behavioral paradigm shift has been slow to take hold. According to a recent article in Psychiatric Times  by Walter Brown, a psychiatrist at Brown Medical School, "Working against the clinical application of [Wehr's and Erkich's] findings is the extent to which they fly in the face of current thinking. The general public seems to regard 7 to 8 hours of unbroken sleep as a birthright; anything less means that something is awry. Sleep specialists share this assumption."

But, Brown wrote, this is changing. Clinical psychiatrists are finding that if they can make their insomnia patients stop seeing their sleep as problematic, their condition becomes more tolerable. "If they perceive interrupted sleep as normal, they experience less distress when they wake at night, and fall back to sleep more easily."

In other words, if you wake up in the night, don't worry about it. "Waking up after a couple of hours may not be insomnia," wrote Wehr. "It may be normal sleep." Ekirch added, "If people don't fight it, they'll find themselves falling asleep again after roughly one hour."

This article was provided by Life’s Little Mysteries, a sister site to Follow Natalie Wolchover on Twitter @nattyover.