Internal Time: Chronotypes, Social Jet Lag, and Why You’re So Tired
Page 21
On several occasions in this book, I have described the difficulties people have in taking seriously the body clock and its biological impact on our lives. These include the elderly professor at the medical school, who thought the body clock was surely only important for sensitive people; or those who insisted that one could readily adapt to certain working hours if one only showed some discipline; or the politicians and teachers who were convinced that young people could not get to sleep early enough to function properly in the morning only because they watched too much television or hung out in discos. Are these critics mostly early types and short sleepers? If so, it is not surprising that the insights of chronobiology are so sluggishly applied in everyday life—the decision makers have no experience with the social jet lag others suffer from, and therefore see no need to change the system. This may be similar to the reason why so many railway stations or public buildings are so poorly adapted for individuals who have difficulty walking—most decision makers can walk perfectly well.7
Napoleon is supposed to have said, “Six hours’ sleep for a man, seven for a woman, and eight for a fool.” Napoleon shared the ability to get by on little sleep with many other leaders—even dictators like Hitler or Stalin. The inventor, scientist, and businessman Thomas Edison claimed that sleep was “a waste of time.”8 But then he apparently also did take frequent naps during the day, an ability he shares with many managers and politicians. Some of them have a remarkable capacity for naps (like cats and dogs)—they can fall asleep within seconds whenever they find a free moment; for example, when sitting in the back of a limousine that takes them from one commitment to the next. It is worth investigating whether the ability for short sleep may be associated with a personality showing tendencies to mania. But not every influential personality regards sleep as a waste of time. Einstein apparently required at least ten hours of sleep for optimal performance.
I once witnessed a good example of sleep phobia coupled with mania at a sleep conference. A physicist had somehow convinced the organizers to offer a talk about the “myth” of having to sleep eight hours. In his lecture he tried to convince his audience that we have become mollycoddled and somehow brainwashed into thinking that we need on average seven to eight hours of sleep. He claimed that we could, with the help of a short training program, reduce our sleep-need without deleterious consequences to about three hours. In doing so he ignored all knowledge about the biology of sleep and its genetic basis. Any biological quantitative trait has a distribution in a population. In denying that sleep duration has a genetic basis, he might just as well have claimed that sleep is not a biological trait in the first place. A paper published in the journal Science in August 2009 clearly shows that some short sleepers have a mutation in one of the clock genes.9 This proves that there is indeed a genetic basis for individual sleep duration. The sleepless physicist was so wrong! The next morning at breakfast, he provided living proof of the falseness of his claims. I witnessed an encounter between him and a waitress: “I can definitely tell caffeine-free coffee from the real thing, and what comes out of this pink thermos is pure decaf, although the label claims it’s coffee!” He was beside himself. Which brings us to the relationship between the body clock and personality or behavior.
Among the first functions to go as a consequence of sleep deprivation are the social skills—an agreeable personality obviously depends on good sleep within our personal circadian window. Most of us have experienced this with young children. When they haven’t slept, their behavior is remarkably obnoxious—so bad in fact that inexperienced parents, relatives, and onlookers might entertain the notion of taking them to a psychiatric unit. Once they have slept, they revert to their angelic selves once again. This is exemplified by a fascinating medical case involving a severe but fortunately rare genetic disorder. Smith-Magenis syndrome is responsible for a multitude of symptoms: mental retardation, malformation, and congenital anomalies, but also severe behavioral pathologies.10 Smith-Magenis children are extremely difficult to handle, have frequent temper tantrums, are extremely tired during the day, and have difficulties sleeping at night. Scientists recently discovered that some Smith-Magenis patients, especially those with the behavioral symptoms described above, have an inverted melatonin rhythm. Normally this hormone is produced at night, but in these patients, it comes up during the day and is absent at night. French pediatricians treated Smith-Magenis children with a beta-blocker in the morning and supplemented melatonin in the evening, thereby reverting the abnormal melatonin rhythm back to its normal phase.11 The result of this treatment was quite spectacular: many of the behavioral problems disappeared, and the children were able to sleep better at night. What was originally thought to be a direct symptom of the genetic deletion turned out to be more of an indirect symptom of children who were forced to sleep and be active at the opposite ends of the twenty-four-hour day. Think of how other children would behave if we wanted them to be active all night and forced them to sleep only during the day.
So chronotype and the individual need for sleep apparently have an effect on our lives and our choice of career; in addition, they seem to be associated with personality and behavior. But which is cause and which is effect? Does personality lead to a certain chronotype or sleep type, or does chronotype or sleep type lead to a certain personality? Several studies have investigated possible associations between time-of-day preferences and personality traits such as extroversion, agreeableness, conscientiousness, neuroticism, or openness.12 Some of these studies found that morning people are generally more agreeable and conscientious while evening people appear to be more neurotic, extroverted, and innovative. Association studies have also looked at sleep duration and found that longer sleep makes individuals more agreeable.
If we were to take the results of these association studies at face value, a clear picture seems to emerge. Late types are exciting individuals, albeit a bit grumpy, extroverted, and neurotic, especially when young, and on the whole they belong to the more innovative members of society. In contrast, early types are very agreeable creatures, reliable and conscientious, but, alas, a bit boring.13
Categorizing humans into boxes based on associations is an old but rather dubious pastime of scientists; it goes back to Hippocrates, who classified human temperaments into four categories based on different body fluids.14 Another typologist was Ernst Kretschmer, who used the physical appearance of the body to classify human temperaments.15 The trouble with all these classifications is that they are a priori based on existing prejudices. Hans Eysenck was the first scientist to try to eliminate the influence of prejudices by using a more scientific approach based on statistics.16
People with certain daily preferences (not equivalent to a chronotype determined by the actual times of sleep and activity) are loosely associated with certain personalities, but such associations may be based on a circular process. People who have a strong urge to comply with social rules, for example, would be biased when filling out a questionnaire probing their daily preferences. They might hesitate to admit that they are late types (under the pressure of public opinion) and would thus artificially create an association between personality and morningness-eveningness type. Do late types appear more innovative than early types because they were, for example, more challenged in school than early types, and always had to invent clever strategies that helped them perform despite not being on top of things in the morning? Do early types choose standard, less innovative careers because they had better grades at school? The fact that early types are more agreeable than late types could merely reflect their low level of social jet lag. As chronobiologists, we would at least demand that personality questionnaires be filled out at different times because the answers would surely depend on time of day.
There are far too many different proffered explanations for the associations found between daily preferences and personality; this is always a sign for a lack of understanding about what the results mean. More, and more rigorous, research is required, using a
pproaches that can tease apart the many potential influences and causalities. Any such investigation should start with an assessment of chronotype based on actual sleep and activity or even more objective measures, such as daily melatonin profiles, instead of the extremely subjective morningness-eveningness preferences.17 It should also take into account at what time of day the questionnaire was filled out in relation to the subject’s chronotype. Finally, such a study should note a subject’s profession, and whether or not the career was chosen fairly freely. My prediction is that if associations between chronotype and personality exist, they may be quite indirect, depending on how different chronotypes fit into the temporal demands of society. Imagine we had the following combination of four individuals, who all shared the fictional Dr. Skinter’s talents, but who differed in their chronotype and in the profession they chose (or were forced to choose): the first might be an early type who has become a neurosurgeon; the second, an early type who has become a jazz musician; the third, a late type who has become a neurosurgeon; and the fourth, a late type who has become a jazz musician. How would these four fill out the questions of the Big Five Inventory? The analysis of such a combination of cases would allow us to tease apart whether personality is directly linked to chronotype or whether their association is an indirect result of how different individuals fit into the temporal demands of society.
24
The Nocturnal Bottleneck
The three older boys always thought that their slightly chubby, red-haired, freckled-face little brother Leon was a bit peculiar because he seemed more interested in strange self-experiments than in kicking a ball with them in the garden. Leon’s most recent project was to find out how the brain works. It had started last Monday when Professor Mallet, the father of a classmate who was a scientist at the local university, had talked to their biology class about how the brain functions. From that day on, Leon had been looking at the world through different eyes. He began to think of many projects that might help him to see how his brain sees the world. The fact that he could use his brain to think about himself intrigued him.
He started to investigate optical illusions that fooled the brain into making mistakes. Lines on paper could be absolutely parallel—Leon established this with a ruler—but if other lines were drawn across them at an angle, they stopped looking parallel. Professor Mallet had told them that if only one single step of a flight of stairs were of a different height, people tended to trip. The brain registered the height of the first few steps and then assumed that the heights of the others would all be identical. Leon entertained the idea of secretly changing the height of one of the steps in their staircase at home by sliding a thin board under the carpet; but since not only his brothers but also his parents frequently ran down those stairs, he quickly abandoned the idea.
He took to closing his eyes whenever he went into a new room, trying to “see with his inner eye” all of its facets. Where was the door? Where were the windows, the table, or any other object he could remember? One day he designed an obstacle course in the basement playroom that was to be experienced in the dark. This time his brothers were interested enough to join in his antics, but with the lights switched off, they stumbled into things. And so for once Leon beat them in every single trial.
On the weekend he visited his aunt. When he woke up in the morning, he was completely confused—the sort of confusion you may also have experienced. He went to look out the window but was confronted with a wardrobe. He looked at the door, but there was only a solid wall with a rather silly picture of a flowerpot. Where was he? What was going on? After what seemed like a small eternity, he suddenly remembered that he wasn’t in his own room at home but in his Auntie Gwendolyn’s guest room. Immediately everything fell into place. Another project had fallen into his hands! What tricks had his brain been playing on him? Had it expected to wake up in his bedroom at home? Why had it taken so long to recognize its mistake? Now that his confusion had cleared, he closed his eyes and was able to “see with his inner eye” both his own bedroom and Aunt Gwendolyn’s guest room.
Back home, Leon’s thoughts became more ambitious. He reflected on something else Professor Mallet had told them: the brain had quite a stubborn character of its own and wasn’t just prepared to passively see, hear, feel, or smell. It could never look at something without some kind of prejudice. He realized that the world around him was somehow etched into his brain. The brain must be constantly building an internal model of its surroundings—similar to the paper models of ships or houses he had built. This made excellent sense, thought Leon, because the brain could then focus only on the things that changed. This gave his brain more time to do other things, like thinking about itself.
Leon eventually got bored with investigating how his brain dealt with space and started to explore how it dealt with time. His grandfather had given him a really cool wristwatch for Christmas—his first—and Leon wore it day and night. Previously, Leon had always seemed to know the approximate time of day. Now he caught himself constantly consulting his watch. He might calculate, for example, that there would still be enough time to finish his homework before leaving for his violin lesson, but then he would immediately forget, and he would check the time on his wrist every five minutes. He decided to leave the watch on his desk and try to consciously teach his brain to estimate time as accurately as possible. He was particularly proud to find that he could even get the time right during the night but then realized that he always awoke at the same time—between 2:30 and 3:00 A.M. He tested whether he could make himself wake up an hour earlier and was surprised that he succeeded after only a few attempts. But there was something puzzling about how his brain figured out what time it was in the night. He never had a good sense of how long he had been sleeping, so obviously his brain didn’t judge the hour by how much time had passed. It just knew! He wondered if it created a similar mental model of the twenty-four-hour day as it obviously did for space.
This book is about clocks. Not about those you can buy, wear, or hang on a wall, but about the clock that ticks away in your body. These were the first two sentences you read in this book, if you are the type who actually reads introductions (I must confess that in most books I don’t). You might therefore have wondered why I now, in the last chapter, tell a story that predominantly deals with space. But as you will see in this chapter, space and time have a lot in common (well beyond Einstein’s space-time continuum). The reason for calling our body’s daily timing system a clock is obvious: its function appears to be measuring time of day. The word clock has very much shaped the way chronobiologists investigate this phenomenon (all our thoughts are strongly shaped by the language we use), but it is a severe reduction of the many services that our internal timing system provides.
By analogy, we could call the capacity of the brain to create a virtual copy of the space around us—the one Leon was so fascinated by—a compass. Our brain’s virtual space machine can certainly be used as a compass because it helps us orient ourselves in space, even if someone switches off the lights.1 But there is much more to this virtual space machine than orientation: the entire structure of our environmental space is kept in working memory. Leon and his brothers raced through an obstacle course in a dark room after having memorized all its structural details. This ability obviously didn’t incorporate only knowledge of the direction of the finish line (such as a compass would provide) but also the exact locations, distances, sizes, and shapes of the obstacles.
The body clock is a similar representation of a temporal space, a Zeitraum.2 In this case it represents the Zeitraum day. In several functions, the body clock is used as a compass as well as a clock. In a wonderful series of experiments, the late Eberhard Gwinner was able to demonstrate how migratory birds use their body clocks for navigation during their long voyages.3 He kept birds in big aviaries exposed to the natural environment. During their migratory season, he placed birds at regular intervals in funnel-shaped cages with walls of paper and an inkpad at the narrow
bottom. After an hour or so, he had a perfect record of the direction in which the bird was predominantly active. All he had to do was to analyze the density of their black footprints on the paper. The results were stunning. While the free members of the species flew from Bavaria to the South of Spain (on their way to their Central African breeding grounds), the experimental birds that were kept behind in the Andechs aviaries hopped predominantly in a southwesterly direction. Once their free conspecifics had crossed the Strait of Gibraltar, the experimental birds also changed their direction toward southeast—as though they were now flying toward Central Africa. We don’t have to presume some esoteric communication between the actual travelers in Spain and the virtual travelers in Andechs because the birds have an internal program that tells them to fly approximately x days in one direction and then y days in the other. The virtual travelers were able to copy the behavior of the actual travelers only if the top of the funnel-shaped cage gave them a free view of the sky. So they obviously oriented themselves by using the sun (or the stars) as a compass. Of course, the sun and the stars don’t stay in one place, because our globe turns. To be able to meaningfully translate the position of a celestial object into a constant direction, the birds had to know the time, and Gwinner was able to prove that they use their body clock for this task. He took some birds out of the natural aviary and kept them for a week in a room where he resynchronized their body clock to an artificial light–dark cycle that was out of phase with the local Andechs day–night cycle. When he then placed the birds into the funnel-shaped recording device with free access to the local sky, they hopped in the wrong direction (although it would have been the right direction according to the resynchronized body clock).4