This undeniable sense of timing over which we have been puzzling is as inherent in humans as it is in all the lower organisms; but, paradoxically, knowing ourselves appears to be more difficult than understanding how other life forms express their time sense. The extent to which human time is either biological or psychological seems to be the root issue.
More than a hundred functions are known to oscillate within our bodies each day, and we share most of them in different degrees of variability with all mammals. At the short end of the time spectrum, there is the ‘/lo-second oscillation of brain waves on an electroencephalogram, the 1-second basic cardiac rhythm, the 6-second respiratory cycle, and the various sleep stages leading up to the 24-hour sleep-wake period. Our body temperatures also fluctuate on a twenty-four-hour cycle, being highest toward evening and lowest toward morning. At the longer end of the time scale lies the 28-day menstrual cycle and the vestige of a 365-day cycle of hibernation.
Physiologists have their own language for dealing with these phenomena, and its electronic metaphors (italicized in this and the next paragraph) suggest how they think our bodies and brains operate. A pacemaker is some entity within the body which is somehow capable of sustaining oscillations and entraining them to other bodily rhythms. Initially, the forced oscillations, or zeitgebeis, are received through the auditory or visual channel before being picked up by transducers or receptors within the body. Though these transducers acquire the time-based information from nature's cues, the pacemaker is capable of measuring time in the absence of these cues. It acts as a kind of alternating-current device which sends out measured pulses of information. Biologists have adopted this information-processing model as a way of explaining biorhythms in the higher life forms. They formulated it on the basis of experiments with whole animals, deprived of normal environmental conditions. The tactic consists of manipulating environmental parameters and then recording the resulting behavior of successive generations of animals. But when biologists try to pinpoint the location of the pacemaker within the organism, things get really complicated. Here the strategy, like that of most scientists, is to divide and conquer. When they examine the rhythm of isolated body parts (the heart taken out of body, liver cells, or blood cells), they discover that the rhythms of the whole animal are really comprised of multiple oscillations, all running together harmoniously in a perfectly tuned condition in the normal specimen. But are all the rhythms in an organism driven by a single pacemaker, or is there a hierarchical organization of pacemakers? And, if the latter, where is the command center in this metaphor that says, "My body is a computer"?
Most experimental biologists believe the major pacemakers in mammals are found in the hypothalamus area of the brain—at the top of the brain stem, directly behind the eyes, to be exact. The site is a tiny bundle of nerve cells known as the suprachiasmatic nuclei (SCN). In certain instances, the information channeled from entryway to command post even can he traced—from the eye, which receives the zeitgeber, along a neural network to the hypothalamic area—by the injection into the retina of substances that can then be followed by radiographic detection techniques.
By studying the free-running behavior of rats after subjecting them to every conceivable manner of sensory deprivation—such as blinding; removal of adrenal gland, pituitary gland, gonads—biologists find no disruption of normal activity. Only when one begins to make knife cuts in the selected areas of the brain in the hypothalamic area does a rat begin to alter its feeding and drinking rhythm drastically from the normal oscillatory cycle. Lesions that destroy the SCN appear to affect the rhythmic discharge of certain chemicals from the adrenal glands, which control eating and drinking behavior as well as the wake-sleep cycle.
In humans there is no circadian cycle more dramatic than the alternation between being awake and being asleep. We are so conditioned to passing through that unremembered instant between the drastically opposed states of consciousness and unconsciousness that any attempt to deprive us of the ability to do so is doomed to failure. On the average, we spend one third of each earth rotation asleep (by contrast, a donkey spends one seventh; an opossum, four fifths); and.
Of course, we do it during the dark period. On the other hand, anyone who forgets to "put out the cat" hecomes painfully aware that felines undergo alternating houts of sleep and wakefulness, each one to two hours long. The animal "catnaps," paying no heed whatever to the day-night cycle, a condition to which it may have adapted as a result of its predatory nature. The opossum, hy contrast, is awake only during morning and evening hours.
It is extremely difficult for a human being to remain awake continuously for more than two or three days. The extreme case recorded in the Guinness Book of World Records is eleven days for a seventeen-year-old boy who achieved his pinnacle only with constant prodding from the outside and with a special motive in mind—to set a world record.
Our strict entrainment to the earth's diurnal period is far-reaching. When you pass a night without sleep, though you are feeling extremely fatigued during the working hours, you quickly get back on track with the normal daily cycle. By midmorning, even though you may be struggling through the twenty-seventh consecutive hour of wakefulness, somehow you feel better than you did while getting through that miserable twenty-first hour during the pre-dawn or the dark part of the diurnal cycle. However, by the time you see the sun go down for the second time, you pay the price twice over by falling into a stupor. Like the experimental animals I discussed earlier, when we depart from the normal conditions of the environment, we struggle to compensate internally.
Just like the free-running rats, people deprived of the normal light-dark cycle by living in caves, or by being confined to sealed rooms under conditions of constant light, become disentrained after a couple of weeks. They alter their biological sleep-wake cycle to a period somewhat different from the usual 8- to 16-hour cycle. Psychologically speaking, they lose all track of time, often believing they have been awake far longer than has actually been the case. They even wonder whether they have slept long enough; but these concerns usually vanish after an extended period of isolation. One subject who was kept in a closed room for several weeks stated, "After a great curiosity about 'true' time, during the first two days of bunker life, I lost all interest in this matter and felt perfectly comfortable to live 'timeless.' 1
We know that the percentage of time we sleep decreases with age; it begins as a prolonged affair but is nonetheless fitful in infants (as any parent knows, it takes about two years before a child is fully fixed into the nocturnal sleep cycle). In old age, somnolence becomes fitful again.
Sleep is not just a single state. Rather, it seems to be made up of subsidiary rhythms. From the rapid eye movements that occur during the time that we dream, we call this period "REM sleep." We spend about one quarter of our sleep time at it. During REM sleep, there is no muscle tone—and fortunately we cannot act out our dreams as we dream them. During non-REM sleep, we do most of our tossing and turning. Muscles are active, but the frequency of brain waves slows down, passing through various substages. These two cycles, REM and non-REM, alternate on approximately a 90-minute cycle throughout the night in all human beings.
This "ultra-dian" oscillation cannot be connected with any known period in the environment; but like all other behavior during the sleep cycle, it seems to be controlled by the interaction among clusters of nerve cells at several sites within the brain, most of them within the brain stem. One such cluster seems to play a role in REM, and another in non-REM sleep. Motor inhibition, or loss of muscular control, is operated by a third area. We know of these connections because the stimulation of these areas results in one or another reaction in the sleeping patient. But still unanswered is the fundamental question of exactly how changes in the activity of different brain-stem nuclei alters the overall activity in the entire central nervous system.
While the human sleep cycle is circadian, the reproductive cycle is predominantly ciica-lunar and chc-annual. Indeed, the very word menstrual is derived from the menses, or month (a contraction of moon-th). At 28 days' length, the menstruation cycle lies, probably not by coincidence, close to the moon's period. In practically all primates, the production of estrogen, variations in bodily temperature, the time of ovulation—all are controlled by a period that spans the interval 25 to 35 days. In southern California, the running of the grun-ion is a major event during the period of the year when the highest tides occur. At either the dark or the full-moon phase of the monthly library to dormitory (and other places) long after the sun has left the sky—hardly photoperiodic creatures.
Cycle, millions of females of this slender species (about the size of a big sardine) suddenly turn up on the beaches. They perform a vertical wiggling dance, burrowing their tails and half their bodies in the sand where they deposit their eggs. The males coil round them and eject sperm to fertilize the eggs. The hole is then covered with wet sand. Two weeks (or half a lunar cycle) later, the eggs hatch, just in time for the young offspring to be helped by the advancing tide back into the ocean. These moon-based cycles may be a reflection of our original ascent from the sea. Remember the oysters? Many feedings and reproductive cycles of organisms that inhabit the ocean receive their input signals from the tidal period which, in turn, is regulated by the position and appearance of the moon.
Half a billion years ago, the moon lay closer to the earth in its orbit. It has been receding ever since; and as a result, the month, measured by the period of revolution of the moon about the earth, has become longer. There is some evidence that when the lunar period was measurably different, some 350 million years ago, living organisms were entrained to that period. During that time, the so-called Middle Devonian period, the month was about a day and a half shorter, making for more months in a year—closer to thirteen, rather than the present twelve. The evidence shows up in fossil coral embedded in sandstone laid down during the Middle Devonian period. Paleontologists have examined the growth ridges in coral which are deposited on both a lunar and an annual "breeding" cycle. During the summertime, when the waters are warmer, the thickness of the chalky deposit is larger than during the winter, when the organism lies in a more dormant state. From a statistical point of view, the data suggest that the coral time clock was ticking according to a different beat than today, one that reflects the natural environment that once existed but has since undergone gradual change (see figure 1.4).
We are a long way from relating the behavior of 350-million-year-old fossil corals directly to the estrous cycle of twenty-first-century women, but the moon's rhythm could have some bearing. For early humans, the time around the full moon offered an extended period of light during which limited hunting and gathering could take place. And under the difficult conditions in which they must have lived, our prehistoric ancestors needed all the light-time they could get. It is reasonable to suppose that the dark-of-the-moon period might have been given over to sedentary activity, which included mating. To use the
FIGURE 1.4 An archaic month calendar frozen in time. A count of the horizontal growth ridges deposited during full-moon phases by this coral from the Devonian period (350 million years ago) indicates that our months once were shorter and there were 13 of them in a year instead of 12. Source: Courtesy Colgate University Department of Geology. Photo by Warren Wheeler.
Physiologist's lingo, this kind of lunar entrainment could have led to the development of hodily pacemakers that controlled the secretion of chemicals related to the breeding cycle. In other words, our brains constructed a lunar-analogue time clock.
The year-clock is evident in mammals that seem to have developed an annual reproductive cycle that ensures that the young will be born at the most promising time for survival; this is particularly true of species residing in temperate climates, where the change of seasons is more dramatic than in the tropics. Animals precisely measure changes in the daily light period. For example, the testes of hamsters remain in regression during the long winter period leading up to the spring equinox. Then, all of a sudden, the weight of the testes is multiplied by 10 in just a few days. This dramatic change seems to take place just when a light period of between 12to 12!4 hours per day is reached. It is as if the critical day length around the equinox triggers a switch within the animal whereby it anticipates the mating season and rapidly begins to prepare for it.
Now, all of this talk sounds distinctly exogenous, though most animal biologists believe that the time clock is switched on by an endogenous circadian system, something like that advocated in Btinning's two-phase hypothesis. Thus, on long days light would tend to fall on a greater portion of the cycle that would stimulate a physiological response. In fact, experiments conducted in the laboratory showed that when a light pulse fell on one of the "light-loving" phases of the 24-hour cycle, the testes became fully functional.®
The circadian clock is blended in with the lunar and the solar reproductive cycles in many other ways. For example, spontaneous births in humans increase dramatically between 3 and 4 a m.; and the estrous cycle and the time of ovulation follow a 24-hour subperiod within the menstrual cycle. But what coordinates these periods?
When freed of all time cues, humans, as well as animals, seem to go through a series of stages or adjustments before becoming totally desynchronized. They behave as if two coupled pacemakers that once provided information to one another had become totally decoupled. There seems to be an attempt on the part of the organism for the first few weeks to imitate the normal period. Then, when a certain phase between two different bodily cycles, such as body temperature and sleep-wake rhythms, is reached, the organism becomes "trapped," and a new oscillation period is settled upon and followed for both. Finally, after a few more weeks, as if a switch were pulled, the pacemakers become totally desynchronized, and the two periods start to drift rapidly away from one another. In each instance, the coupled pacemakers appear to be contained within or close to the SCN neural clusters. Thus, the human clock, at least as we conceive it, seems to behave much like a machine.
We've got rhythm! There's no doubt about it. And our desire to memorize, mark, and record it—the subject of the next chapter— seems to have been bred within us right from the start, before we could even call ourselves human. Like all living organisms, we march in time to the dependable beat impressed upon us by nature's background music. For us today, that environmental combination of tunes happens to consist of a 24-hour alternating light-dark point-counterpoint, backed up by the song of the moon, sung both in different time and in a different key. The sun chimes in with its second chorus; the music of the seasons sounds with a much lower pitch that conflates with the lunar beat to produce a kind of disharmony. All creatures of the earth automatically tap their extremities to keep time with its tunes. Ours is a beat that could have been different had the earth's past history been otherwise (see figure 1.4). Why not a 5-hour day, a 100-day year, and maybe 3 moons in the sky instead of 1 ? Different planet, different music. But this is the way it is here and now, at our place.
World History
