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By Root 946 0

½-hour cycle: his internal clock had not shifted, only his conscious understanding of time. But Siffre parsimoniously meted out his rations to himself, for in misunderstanding the length of his day, he believed to the last that he had weeks more to endure. At the fifty-seventh awakening—the final day of the experiment—Siffre believed it to be August 20 when, in truth, it was September 14: the time graph he'd kept lagged twenty-five days behind the actual date. "I underestimated by almost half the length of my working or waking hours; a 'day' that I estimated at seven hours actually lasted on the average fourteen hours and forty minutes," he commented after his emergence from the cave.

Siffre's experience proved that our circadian rhythm may be able to withstand the periodic absence of light, but additional research since then suggests that even small amounts of artificial light can significantly disrupt that rhythm. The effects of artificial light on sleep are particularly profound, for it is the absence of light that induces our biological clock to signal the pineal gland to increase production of the sleep-inducing hormone melatonin. Although bright lights are difficult to separate from other things that may contribute to troubled sleep—noise, coffee, busy evenings—Dr. Charles Czeisler, who conducted a study of human response to light at Brigham and Women's Hospital in Boston, found that not only intense artificial light but also long periods of lower-level artificial light can disrupt the human biological clock. As a result, the clock can be shifted by up to four or five hours, "meaning that most people in the United States are actually on Hawaii time. Instead of people experiencing a peak drive for sleep between midnight and 1 A.M., for most people this is now at 4 A.M. or 5 A.M. ...[They] are forced to wake up earlier than they would like to and remain tired during the day." Dr. Czeisler notes, "Every time we turn on a light we are inadvertently taking a drug that affects how we will sleep and how we will be awake the next day."

Additionally, in modern industrial societies, humans tend to give themselves little time to wind down in darkness and quiet before attempting to go to sleep. And they no l onger vary their sleep according to seasonal changes in the length of days and nights, although even now the human biological clock still shifts according to the season and the amount of sunlight in a day. For instance, in the north temperate latitudes, the biological night is long during the winter and short during the summer, but people often bathe themselves in sixteen hours of light during all seasons of the year, as if every night fell during high summer.

Even the eight hours of uninterrupted sleep now considered desirable may be something imposed by industrial society, which requires every day of the year and all hours of the day to be divided in a certain way: now work, now relaxation, now sleep. Historian A. Roger Ekirch discovered that medieval villagers slept in a different way from modern people. Each night, they experienced divided sleep. They would go to bed soon after sundown, sleep for four or five hours—this was called "first sleep"—and then wake up an hour or two after midnight. Some people inevitably took advantage of the early-morning hours to get out of bed and work: students bent over their books; women did housework they couldn't get to during the day. Some even visited neighbors or slipped out of the house to steal firewood or rob an orchard. It was a good time for sex. But frequently people would lie quietly in bed, resting or talking, before they fell back into a lighter, dream-filled sleep—called "second sleep"—that lasted until sunrise. The quiet, free time in the small hours would have been dearly valued in a society where the days were filled with labor and obligation.

Divided sleep, Ekirch notes, began to slip away as artificial light increased. By the seventeenth century, the wealthy, who already prized their nightlife, no longer experienced it. Later, as the middle class acquired increased