The Winged Chariot of Time

Resetting man’s internal rhythms

In man, as compared with plants and other animals, the biological clock is rather poorly developed. The human animal is weak in instincts as it is powerful in intelligence — and the instinctive time sense is no exception.

Nonetheless, man does show certain circadian cycles that parallel those of the lower animals. Chief amongst these is the rhythm of sleep and waking. Indeed, barometers of circadian cycles that have been studied — daily variations in hormonal secretions, urine production and body temperature — all seem, under normal conditions, to keep step with sleep-walking cycle and are probably related to or governed by it.

Man’s sleep-waking cycle seldom runs exactly 24 hours: An English cave expert who spent 105 days alone underground found that he tended to fall asleep a little later every night; his internal “day” averaged 24.7 hours. In some cases, the cycle can be experimentally reset to a slightly different period. Nathaniel Kleitman, an American expert who has studied sleep from many standpoints, proved this during a stay of several weeks in Mammoth Cave, Kentucky, with an associate, Bruce Richardson. The two scientists put themselves on a 28-hour day for 32 days. Their body temperatures, which seem to be the most useful index of the body’s circadian changes, were recorded at frequent intervals.

Richardson adjusted to the abnormal days with no difficulty; his temperature regularly hit a peak during waking hours and a trough while he slept. Kleitman’s body, however, clung stubbornly to its 24-hour-day cycle; as a result, he was frequently sleepy and irritable during the day and restless at night.

Man’s sleep-waking cycle, whatever its individual idiosyncrasies, does not appear immediately at birth. As every parent knows too well, a newborn baby has no fixed pattern of sleep; it will awaken, and demand food, at times that clash with its parent’s sleep patterns. The baby seems to learn its cycle from its parents slowly, over the course of many weeks- in contrast with most cycles in the plant and animal worlds, which require only a day or two to establish themselves.

Man’s internal clock remains flexible through life, adapting easily to gradual change. As early as 1907, an Engilsh doctor on a voyage from Australia to England noted that his cycle of temperature variation kept pace with the ship’s passage through different time zones. It regularly reached its daily maximum at 6 p.m. local time, though by the time he reached London, this corresponded to 4 a.m. Melbourne time.