This is not to minimize the torment of insomnia over the course of a restless night. But for most of us, slumber is reasonably tranquil -- especially when compared with what passed for a night's rest before the modern era. Despite nostalgic notions about sleep in past centuries, threats to peaceful slumber lurked everywhere, from lice and noxious chamber pots to tempestuous weather.

Worst in this pre-penicillin age was sickness, especially such respiratory tract illnesses as influenza, pulmonary tuberculosis and asthma, all aggravated by bedding rife with mites. One 18th-century diarist recounts that asthma forced her husband to sleep in a chair for months, with "watchers" required to hold his head upright. Among the laboring poor, whose living conditions were horrendous, sleep deprivation was probably chronic, prompting many to nap at midday, much to the annoyance of their masters.

As if these maladies were not enough, 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. The Harvard chronobiologist Charles A. Czeisler has aptly likened lighting to a drug in its physiological effects, producing, among other changes, altered levels of melatonin, the brain hormone that helps to regulate our circadian clock.