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The understanding of the way artificial light adversely affects living things is still an unfolding mystery, but we do know that ubiquitous light wreaks havoc with our circadian rhythm—our daily cycle of variations in body temperature, hormone levels, heart rate, and sleep-wake times that is controlled by our biological clock. In humans, as in all mammals, the clock consists of a small cluster of nerve cells in the hypothalamus, which is cued by the varying levels of light that reach it from the retina. Having evolved in the absence of artificial light, it's broadly attuned to sunrise and sunset.

Researchers once believed that people living in modern industrial societies might have evolved away from the human biological clock as it functioned in earlier times, for the workings of our internal clock aren't always obvious, divorced as we are from the environmental constrictions of days, months, and years: we control our heat and light, and we no longer breed on a seasonal cycle. However, the experience of French geologist Michel Siffre, who in the summer of 1963 descended into the Scarasson Cave—a glaciated cavern under the French-Italian Maritime Alps—where he spent more than two months without sun, helped to confirm that our internal clock continues to keep time independent of our modern way of life.

Siffre set up camp alongside an underground glacier, surrounded by the corrosions and dissolutions of the cave, its dripping darkness and cold. Though he had one incandescent bulb to see by, he had no way of knowing the hour. "I wanted to investigate time," he explained, "that most inapprehensible and irreversible thing. I wanted to investigate that notion of time which has haunted humanity since its beginning." He reckoned his days by awakenings, recording each in a diary, and also telephoning scientists on the earth's surface, who registered the actual hour of his call, though in conversation they never told him what day it was or the time of day.

During his months alone in the dark, he had to cope with isolation and loneliness, and with the threats from the unstable walls and ceilings all around him. "This morning I was completely stunned," he wrote, after hearing a series of loud cave-ins of rock and ice. "My pulse was rapid, my mind full of dark thoughts. In such moments one realizes one's insignificance.... Birth, life, death, and then—nothing. No, no! Birth, creativity, and death—that sums up a man; the rest belongs to the animal kingdom. When I had partially recovered from my fright I looked at myself in the mirror: a pale and puffy face, with haggard eyes brimming with tears stared out of the glass."

The anxieties, confusion, and physical stress of those months would take their toll. "I emerged," he would say later, "as a half-crazed, disjointed marionette." Even so, he meticulously recorded his observation of time, and his diary is the record of a man who has lost all comprehension of duration:

Forty-second awakening:...I really seem to have no least idea of the passage of time. This morning, as an example, after telephoning to the surface and talking for a while, I wondered afterward how long the telephone conversation had lasted, and could not even hazard a guess.... Fifty-second awakening:...I am losing all notion of time.... When, for instance, I telephone the surface and indicate what time I think it is, thinking that only an hour has elapsed between my waking up and eating breakfast, it may well be that four or five hours have elapsed. And here is something hard to explain: the main thing, I believe, is the idea of time that I have at the very moment of telephoning. If I called an hour earlier, I would still have stated the same figure.... I am having great difficulty to recall what I have done today. It costs me a real intellectual effort to recall such things.

During Siffre's months underground, the scientists on the surface keeping track of his daily cycles of waking and sleeping saw that they remained quite near a 24