By A. ROGER EKIRCH
Published: February 19, 2006
we now also know that pre-industrial families commonly experienced a "broken" pattern of sleep, though few contemporaries regarded it in a pejorative light. Until the modern age, most households had two distinct intervals of slumber, known as "first" and "second" sleep, bridged by an hour or more of quiet wakefulness. Usually, people would retire between 9 and 10 o'clock only to stir past midnight to smoke a pipe, brew a tub of ale or even converse with a neighbor.
Others remained in bed to pray or make love. This time after the first sleep was praised as uniquely suited for sexual intimacy; rested couples have "more enjoyment" and "do it better," as one 16th-century French doctor wrote. Often, people might simply have lain in bed ruminating on the meaning of a fresh dream, thereby permitting the conscious mind a window onto the human psyche that remains shuttered for those in the modern day too quick to awake and arise.
The principal explanation for this enigmatic pattern of slumber probably lies in the nocturnal darkness that enveloped pre-industrial households — in short, the absence of artificial lighting. There is a growing consensus on the impact of modern lighting on sleep.
In fact, during clinical experiments at the National Institute of Mental Health, human subjects deprived of light at night for weeks at a time exhibited a segmented pattern of sleep closely resembling that related in historical sources (as well as that still exhibited by many wild mammals). The subjects also experienced, during intervals of wakefulness, measurably higher levels of prolactin, the hormone that allows hens to sit happily upon their eggs for long periods.
These elevations of prolactin reinforce historical descriptions of complacent feelings at "first waking" and, back then, probably helped calm people's worries about the night's perils. Prolactin is also what differentiates segmented sleep, with its interval of "non-anxious wakefulness" that nearly resembles a meditative state
Remarkably, then, our pattern of consolidated sleep has been a relatively recent development, another product of the industrial age, while segmented sleep was long the natural form of our slumber, having a provenance as old as humankind. (Homer even invoked the term "first sleep" in "The Odyssey.")