In the majority of subjects who lived for several weeks in the bunker, all bodily rhythms that were recorded oscillated in synchrony.1 In other words, all had the same period of circa twenty-four hours. In less than a third of the subjects, however, an unforeseen picture emerged: different rhythms oscillated with different periods and, therefore, drifted away from each other. While the minimum core body temperature recurred about every twenty-five hours, the subjects went to bed every forty hours. It seemed as if different outputs of the internal timing system had desynchronized from each other. This observation suggested that more than one clock controls our daily behavior and physiology.
In most of the subjects who showed this internal desynchronization, the sleep–wake rhythm together with the rest–activity rhythm continued at a slower pace than other, more basic bodily functions, such as the waxing and waning of body temperature or of hormones. In a small number of subjects, the desynchronization between the behavioral and the physiological rhythms was the other way around. They lived shorter days in their behavior compared with their physiological rhythms, which in all cases were never far from twenty-four hours. Because the length of the body clock’s day was only close to but not exactly that of our normal twenty-four-hour day, the rhythms produced by the body clock were called circadian rhythms.2 This rather scientific-sounding term even made it into the pop charts in 1998 in the song “Daysleeper” by the group REM.3
Aschoff and his colleagues discovered the internal desynchronization of different body rhythms in the late 1960s. The activity rhythm in animals can adopt quite long or short periods in temporal isolation. These can even split into two distinct components, each running at its own period, but a clear separation between the rhythms in behavior and physiology seems to be a human specialty. Some subjects in the bunker lived through days twice as long as our normal days and in some rare cases even longer. Yet in all cases, the period of the core body temperature rhythm and other physiological rhythms stayed close to twenty-four hours.
Aschoff’s hypothesis of internal desynchronization was often criticized, based on the following argument. In the case of an apparent internal desynchrony, the period of the sleep–wake cycle is often double that of the body temperature rhythm. Consequently, subjects experience two temperature minima during each of their behavioral days, one during their nocturnal sleep episode and another in the middle of their active period. Many of these subjects took extended afternoon naps during their extra long bunker days approximately around the time when their body temperature hit a second, “daytime” low. The critics argued that the nap was in reality a true nocturnal sleep episode, misinterpreted by Aschoff and his team.
Although the “misinterpreted naps” argument is valid and puts the human clock in line with that of other animals, there are several observations that support Aschoff’s hypothesis of internal desynchrony. As described at the beginning of this chapter, the subjects who experienced these extra-long days never mentioned anything unusual in their bunker diaries. They ate only three main meals throughout these long days and didn’t double their portions (which one would expect after a doubled day); in addition, they went on average to the toilet to defecate only once during their extra-long days. The most compelling result supporting the concept of internal desynchronization concerned the passing of subjective time. To monitor how the time-free environment affected subjective time, subjects had to perform time estimation experiments.4 Subjects in temporal isolation behave quite similarly to other isolated individuals whom we all have read about, for example Daniel Defoe’s Robinson Crusoe. They become extremely observant and try to keep track of the time of day and especially of the exact number of their isolation days. So bunker subjects were ideal in that they recorded anything out of the ordinary. If, for example, subjects realized that they had pressed the hour-button not the usual sixteen times per day but as often as thirty-two times, they would have noted this in their diaries. Indeed, Aschoff’s recordings showed that the estimation of one hour went along with the length of the days they lived in the bunker, even if they lasted as long as fifty hours. Their short-interval minute estimation, however, didn’t change along with their days’ length. The stability of the short-interval time estimation offers yet another explanation why subjects had no clue as to the actual length of their bunker days. They would have immediately noticed if their short-time perception had changed. For example, the music they were allowed to listen to should have sounded strange. (“I have the feeling that my record player is on its way out, because it runs fast” could have been an entry in their diaries.)
Despite clock researchers interpreting the results of the Andechs bunker differently, these experiments have introduced us to a fascinating world of internal time. They showed that each individual’s body clock responds quite differently to the time-free environment. Every subject adopted his or her very own individual internal day length, and some even lived by more than one internal time frame, with their physiology “listening” to one and their behavior to another. Aschoff’s notes showed that subjects felt best if bodily functions were in synchrony.
You may have asked yourself why evolution was so sloppy as to create a biological clock that cannot keep track of time properly. Even without internal desynchrony, the internal days produced by the body clock of bunker subjects rarely lasted exactly twenty-four hours. The biological clocks of most plants and animals show similar deviations from the twenty-four-hour day when kept in temporal isolation. Evolution has produced impeccable brain functions, which enable us to learn to hit the center of a dartboard from a great distance. It has selected for fine motor skills, so that we are capable of keeping an exact tempo on a musical instrument. And it has equipped us with auditory skills that can distinguish two sound clicks separated by milliseconds. So, why hasn’t it created a more exact body clock?
One could argue that exactness is apparently not necessary for a good body clock, otherwise it would surely have been the outcome of evolution. Yet this argument is circular since it postulates a perfect evolution. There is actually a much simpler and more logical answer to the question of clock evolution. Ever since de Mairan discovered the ability of biological clocks to run free in constant darkness, researchers have been fascinated by the fact that the daily rhythms are not merely a reaction to night and day but are obviously generated by an internal mechanism. Researchers remain fascinated with biological rhythms continuing unabated in constant conditions, and they still investigate many properties of the body clock in time-free experiments. Yet, organisms never encountered a time-free environment over the course of evolution. With few exceptions, such as cave dwellers or creatures on the bottom of the ocean, organisms have always experienced brighter, warmer days and darker, colder nights. Even these persistent daily changes can vary a great deal over the course of a year in most parts of the world. The biological clock did not evolve in a time-free world, and constant conditions were never part of the selection pressure driving the evolution of the body clock. Thus, evolution’s rules of chance and necessity could not act on the development of a biological clock that continues with a precise twenty-four-hour rhythm in constant conditions.5 But why do biological clocks continue to oscillate without time-of-day information in the first place? This ability merely reflects that the way clocks evolved under an endless stream of daily changes has formed them in such a way that they continue even in a time-free environment.
If I have succeeded in convincing you that time-free conditions show us only how the clock evolved to serve the organism in the real, time-driven world, you may conclude that the bunker experiments described in this chapter are all extremely artificial and have little to do with everyday life. Yet, there are four important points to take away from these experiments. First, our body’s internal day is controlled by its own biological clock (as in most other creatures). Second, because clocks do not generate an exact twenty-four-hour day, they obviously must be periodically set—how else could they be of any advantage in real
life? Third, this internal timing system can differ from individual to individual. Finally, we feel best if all of our bodily functions oscillate in synchrony.
6
The Periodic Shift Worker
It was seven o’clock when the alarm abruptly woke Harriet from deep sleep—she must have drifted off only a couple of hours ago. She angrily pressed the snooze button on her alarm clock and turned around again. She fell asleep immediately but was awakened a couple of minutes later by the alarm’s insistent reminder. This procedure continued for about another half an hour. Although she sensed that it was once again a magnificent summer morning, promising to be a wonderfully warm day, she felt like an ice cube and dog-tired. In her half-conscious state she wondered how appropriate the expression was because her golden retriever, Sally, was obviously already wide awake, giving her cheek a sloppy kiss. When Sally’s morning greetings got too persistent, making it impossible for her to doze off again, she turned off the snooze function, got out of bed, and fumbled her way into the kitchen to get the coffee going while she took a long shower.
Once again, she was living through one of those stages where she couldn’t sleep at night, couldn’t get up in the mornings, and only fully woke up during the afternoon. These stages repeated themselves relentlessly in a monthly rhythm. For approximately two weeks, the symptoms got worse and then gradually improved again for the rest of the month. When she sat at her breakfast table clutching her coffee mug and unable to eat even half a piece of toast, half-heartedly listening to the radio’s morning program, she longed for those one-and-a-half weeks in every month where she slept at night and was fresh during the day. She lost track of time, almost falling asleep again while sitting upright at the kitchen table, when she realized that the 9 o’clock news had already started. After she had gulped down the rest of her coffee—what would she do without caffeine?—Harriet got her bag, put Sally’s harness on, and walked out into the staircase of her apartment building. It took her much longer than on her good days to lock the door with her key. Then she took Sally—or rather Sally took her—down the stairs onto the street. She made her way to the bus stop, and when she heard the bus coming she made a cautious step toward the curb, waited until the doors opened, responded to the bus driver’s cheerful “Good morning, Harriet and Sally,” and took her usual seat. Exhausted, she asked herself why she was so different from those who were otherwise so similar to her.
You may have read that last sentence several times, finding it a bit awkward. The strangeness of this sentence is intentional and holds a clue to the problem discussed here. You may have your own hypothesis to explain Harriet’s problems. As a woman, she goes through monthly hormonal cycles, which could be the cause of her sleeping difficulties, but in this case your hypothesis is incorrect—although it could be an entirely probable solution. Harriet’s sleeping problems are related to her internal timing system.
As you read in the preceding chapter, the body clock of most humans generates days slightly longer than twenty-four hours. For the sake of simplicity, let us assume that the internal day of a bunker subject is exactly twenty-five hours and that his behavioral and physiological rhythms do not desynchronize. Imagine that he lives his entire life in the Andechs bunker but that he has to work for a living as an employee with a local company via the internet. Further, his hours of work are not different from those of other people living outside the bunker. I have described the many precautions that Aschoff and Wever considered when they built the bunker. These precautions were taken because they just didn’t know at the time which environmental factors could influence the body clock, and hence jeopardize the experiments by synchronizing it to twenty-four hours. Now almost fifty years later, we know that we can make the body clock run free by simply keeping subjects in a windowless room, so they do not perceive natural night and day or read the local time from any device. De Mairan, we recall, compared his mimosa plant to sick people lying in bed for days but still keeping to a normal sleep–wake rhythm without ever seeing true daylight. Although the French astronomer discovered the persistence of daily rhythms in constant darkness, he didn’t quantify the rhythms of leaf movements and thus missed the fact that free-running rhythms can be longer or shorter than the twenty-four-hour day outside.
The imaginary subject in his lifelong bunker situation has a problem. Since his body clock’s day is twenty-five hours long while his work days repeat every twenty-four hours, he has to get up every day an (internal) hour earlier to be punctual for his computer appointments. After twelve days his body time is twelve hours later than local time, so that he has to be at work in the middle of his internal night—he has gradually become a night-shift worker. Another twelve days later, his body time will be again in synch with local time, and he can easily wake up to go perkily about his computer work.1
Of course, you immediately recognize the similarity of the symptoms between the subject in the bunker and Harriet. But why is Harriet’s life affected as if she is living in the Andechs bunker? The answer is simple: because her body clock is not synchronized to the social world. But why? You may have had another hypothesis, namely that Harriet is blind and that Sally is her guide dog. If so, your hypothesis is absolutely correct. In the last sentence, Harriet asks herself why she is so different from the many other blind people who sleep as well as she does during her best periods every day of the month.
Light is the main signal that resets the body clocks of plants and animals, including humans, to the twenty-four hours of the earth’s rotation. Because Harriet is blind, her body clock is not informed about night and day and, therefore, runs free, as if she were permanently living in the Andechs bunker. So far so good; but how are we to answer her question?
Until recently, ophthalmologists were convinced that the eye of mammals was one of the best-understood organs in the history of anatomy and neuroscience. Eyes provide our brain with information about the structure of the outside world and, thus, help us and other animals to find food; to recognize enemies, parents, or partners; even to read these lines. Thus, our entire concept of the eyes’ function is focused on vision. Light enters the eye through the lens and, as in a camera, projects an upside-down image of the world onto the back of the eyeball. This part of the eye is covered with a layer of cells, called the retina, which consists of millions of tiny light receptors, sampling the picture. In our electronic age, we could say the picture is translated into pixels. Several of these receptors, after communicating with their neighbors (to increase contrast, for example), assemble the collected information into packages that represent the light quality of a given point on the retina and, thus, of a given area of the outside world. These information packages are sent into the brain via the optic nerve.
We have two eyes to provide our brain with enough information to see the world in three dimensions. The two images encoded by the left and the right eye have to be put together in the correct way so that higher regions of the brain can make sense of the puzzle pieces. Part of the optic nerve, therefore, has to cross into the other half of the brain. This nerve crossing lies a couple of centimeters behind the bridge of our nose, forming the optic chiasm.2
The retina houses two kinds of light receptors, which have been given names according to their shapes. The rods are responsible for encoding pictures under dim light conditions and produce a mental image in grayscale. The cones are responsible for helping the brain to compute color when more light is available. Light receptors are highly specialized; each type can only detect light within a relatively narrow range of wavelengths (light color). If the retina had only one type of receptor, the brain would be incapable of computing colors—it could only create mental images consisting of brighter and darker “pixels.” The reason why we cannot see color under dim light conditions is that our retinas have only one type of rods. The brain can only recognize different colors if it can compare the information coming from at least two different receptor types, each of which are specialized for a different range of wavel
engths but “look” at the same spot in the picture. If one of them is specialized on red light and another on blue, and if we were looking at a strawberry, the red receptor would transmit the information “it’s very bright,” and the blue receptor would convey “it’s pretty dark.” If we were looking at a blueberry, they would provide the opposite information. Color detection works even better with more than two receptor types. Humans have three different cone types, specialized for red, blue, and green, respectively. The reason why some people are color blind is because one or more of their genes, containing the information for building an important component of a cone type, is defective. I am going into the science of vision in detail because light reception is extremely important for understanding internal time.
All this information and much more had been worked out in great detail by the 1980s, and scientists began to understand how vision works—from the first stages of translating light information into nerve signals (in the eyes) right up to the computation of this information into mental pictures (in the cortex).3 The visioncentric concept of the eye received a big shock when Russell Foster, a British clock researcher, asked how light synchronized the body clock. But before I elaborate on why Foster’s findings were so important (far beyond clock research), I will have to take another detour.
Children and clock researchers have a common interest: hamsters and mice. The daily activity rhythm of most rodents can be easily recorded because they love nothing more than running in a wheel. Not only “bored” laboratory rodents, living in captivity, become wheel-running junkies. The story goes that an American clock researcher stored unused running wheels from the lab in his garage. Coming home late one evening, he heard the familiar squeaky sound of turning wheels coming from the garage and found that a family of wild mice had adopted the garage as a workout gym. Most rodents are active at night and sleep during the day, so they run at predictable times every day or rather every night.4 To record a rodent’s body clock, we simply have to hook up the shaft of the running wheel to a little electric contact that is closed once every rotation. In the early days of clock research, the switches were connected to a pen mounted on a slowly turning roll of long paper. Nowadays, the impulses from the electrical switches are recorded by computer programs. The original recording set-up produced broad bands of ink when the pen was jerking up and down each time the contact closed—that is when the rodent was running in the wheel—and a straight, thin line when the animal was asleep. If the paper was moving at a constant speed (for example, one centimeter per hour), all we had to do was to cut the paper strips into pieces of twenty-four centimeters and paste them underneath each other onto a piece of cardboard to produce data like that shown in this chapter.
Internal Time: Chronotypes, Social Jet Lag, and Why You’re So Tired Page 5
