Internal Time: Chronotypes, Social Jet Lag, and Why You’re So Tired

by Roenneberg, Till

  The ability of some people to program themselves to wake up at a certain exact time is another example where the body clock is used in its literal function. Although this ability is not part of the body clock itself, it uses its “knowledge” about time of day. (Leon used this ability to reset the timing of his regular nocturnal awakenings.)

  But the biological timing system can do much more than simply tell the time of day: it internally represents the Zeitraum day. It forms a network of information between biochemical processes at the cellular level and the behavior of the organism. Thereby it creates an internal temporal structure that coordinates all bodily functions within the Zeitraum. This temporal program anticipates the regular environmental changes and prepares the organism to do the right things at the right times of day. The different stages of the day and their different challenges (light and darkness, warmer and colder temperatures, as well as time of day–dependent availabilities of food sources, presence of enemies, and so on) are comparable to the obstacles in the races organized by Leon.5

  Evolution itself is an even more compelling case for the hypothesis that the body clock is not simply a time-of-day teller. As we have already discussed, the rules of evolution are based on (almost) random mutations, which are then challenged by many different selection pressures. An important selection pressure is that of competition for resources. These need not necessarily always be food; sufficient space and opportunity to reproduce and raise offspring are as much resources in the selection process. This is why all living creatures constantly search for new niches offering them potential advantages during their evolution. Conquering a niche often involves genetic changes across a species that allow better adaption to the new environment.6 Pioneers occupying a new niche have an enormous advantage until the competition arrives. We commonly think of these niches as spatial structures. When the first animals left the oceans and survived on land, many adaptive changes had taken place: for example, legs had been selected for over fins, and lungs over gills; the body had to cope with much more weight (no floating around anymore); measures had to develop to prevent desiccation as well as to find and store water.

  Humans are primates, and primates are mammals. The first mammals appeared on earth between 200 and 250 million years ago—a very short time in evolutionary units. To put that figure in perspective, the first primitive unicellular organisms appeared roughly 4,500 million years ago; the first cell with a proper nucleus appeared about 1,500 million years ago;7 and the first animals with bones inside their body appeared on land only 380 million years ago.8 These were the ancestors of reptiles, which started to conquer every livable niche outside of the oceans. But after that, where could one turn when it got too crowded on land? There was one spatial niche left that hadn’t been conquered yet by larger animals: the airspace above, so far colonized only by insects. The first flying reptiles pioneered this niche (the ancestors of the feathered creatures we know today as birds). But there was yet another niche waiting to be conquered.

  Life on land also meant that animals often had to cope with the large temperature changes between cold nights and hot days.9 Everything in a living creature depends on biochemical reactions, and most of them happen faster the warmer it is. This means that reptiles can catch prey and run away from enemies more easily during the daytime, because the biochemical reactions necessary for movement function more efficiently. No wonder that practically all reptiles are day-active. Being day-active means that all aspects of life adapt to the sunlit day, from basic metabolism to the senses and behavior.

  You may have asked yourself what niche could have been left after ocean, lakes, land, soil, and airspace had been occupied. But now that I have mentioned the alternatives of night- and day-activity, you may guess where this chapter is going. This particular hypothesis about how mammals evolved presumes a change not between two spatial niches but between two niches within the Zeitraum day. While birds are the end product of an evolutionary line that conquered the airspace, the ancestors of mammals conquered the night. Our mammalian ancestors went through a nocturnal bottleneck. Developing the ability to fly was a prerequisite to conquering the airspace, thereby happily feeding on insects and getting away from all those ground-bound reptilian enemies. Perfecting temperature regulation (becoming truly warm-blooded) was a prerequisite to conquering the cold nights. During the day our mammalian ancestors could hide in a dark and cool burrow and during the cold nights they could roam around, dodging dangerous but now potentially sluggish reptiles.10 Mammals could keep their body temperature at around 37°C even at night while most reptiles were highly dependent on the ambient temperature—they were simply not competition.

  Changing from one spatial niche to another would not be possible without an excellent internal representation of the environment. Analogously, switching between niches within the Zeitraum day would not have been possible without a comparable internal representation—provided by the body clock. It seems that during evolution changes between spatial niches and those within the Zeitraum day never occurred simultaneously—confirming their equivalent importance for the organism (the long evolutionary adaptation process would not allow the conquering of two independent niches simultaneously). Changing a niche is a long-term prospect, but it is not a one-way street. Many terrestrial animals have gone back into the water and many birds have forgotten how to fly. Switching back to an original niche also happens in the temporal domain. All our ancestors must have been night-active (most mammals still are), but we and some other mammals have reconquered the day. Once the day-active birds had firmly established their dominance in the air, some of them switched to night activity. The lark is a good example of the former and the owl is a good example of the latter. You see, this book is about larks and owls from beginning to end!

  Our internal clock is not only important in making our body do the right things at the right times; it has also been a crucial player in our very own mammalian evolution. In this chapter you have seen the great advantages of flexibility in occupying niches on an evolutionary scale. The wide variety of chronotypes in modern populations potentially provides similar flexibility. Our species has conquered almost every spatial niche on earth. We haven’t adapted to all these niches as organisms do in evolution, however. If we couldn’t carry with us all we need as day-active, land-living mammals, we would not survive. We need torches in the dark, we carry oxygen under water or when the air is too thin, and we heat our homes in winter, to name just a few of our adaptive insufficiencies. Our flexibility within the Zeitraum day is, however, genuine—let’s make the most of the chronotype differences within our species.

  Ann and Jacob would perform so much better if we appreciated that most teenagers have to start school at around their internal midnight. If we could sleep within the appropriate circadian window (like the young man in the bunker, or like Barbara and Gerry—but unlike Sergeant Stein or Timothy) we would be less tired and more cheerful during the day; we would perform better; and we would be healthier. Our work schedules have to acknowledge that most of us are no longer farmers. More flexibility would be advantageous for gathering the proverbial mushrooms or for hunting at night like Urf. They would also allow people like Dr. Skinter a real choice in their professional careers. We would even become more tolerant in our moral judgments about the faster and slower hamsters among us, and spouses, like Louise and Bruno, might be more forgiving of their respective differences.

  Internal timing is genetic, as Sarah and her family would testify. In addition, Harriet would sadly agree that our body clock doesn’t simply respond to social cues but uses its own response characteristics for synchronizing to day and night, light and darkness on earth (and not on other planets). That’s why Germany’s former socialists go to work earlier than their capitalist compatriots. We have to appreciate that industrialization means working inside and that lack of light is problematic not only for our body clock but also for many other aspects of our well-being—think of Hanna, Sophie, Frederic, and Jos
eph. Architects have to find ways to allow more bright daylight into buildings (without increasing our carbon footprint).

  As we are likely to increasingly abuse our body clock with jet travel and shift work, like Oscar and Jerry or Marco and Maria, basic and applied research has to offer solutions for minimizing the detrimental consequences. We need new individualized shift-work schedules. We have to discuss the pros and cons of messing with our body clock as we do with daylight saving time (although our motives should be more constructive than Edgar’s). Internal timing is of ecological importance—from single-cell algae to man—and was instrumental in evolution. It also controls all of our bodily functions and therefore should play a crucial role in medical diagnosis and treatments. Although our curious astronomer acknowledged the wide consequences of his discovery, I doubt that he had the slightest idea of how colossal they would turn out to be.

  Notes

  Acknowledgments

  Index

  Notes

  1. Worlds Apart

  1.A moral from James Thurber, Fables for Our Time.

  2.By definition, the term sex is used to distinguish the difference between a man and a woman based on inherited genes; the term gender is used to indicate differences due to learning, social context, or culture.

  3.Age at the onset of puberty varies among different regions and also depends on diet and cultural setting. Nowadays, puberty could be defined biochemically by measuring certain hormones. Practically, however, more evident signs are used. In girls it is defined as the first menstruation (menarche), in boys by the growth of the testes and the penis. The end of puberty is defined in both young women and men when the arms, legs, and feet stop growing (not necessarily when full body height is reached).

  4.From the Greek word chronos.

  5.Long before scientists set out to investigate different chronotypes in humans, people have noted that different bird species start to sing at very specific times. In Central Europe, for example, the redstart is one of the first to sing before the sun rises, followed in incredibly precise intervals of approximately five minutes by the robin, the blackbird, the wren, the cuckoo, the great tit, the chiffchaff, and so on. Unlike the question we pursue in this book, the individual differences within a species, this “bird clock” shows the differences in chronotype across species.

  6.The Munich ChronoType Questionnaire (MCTQ). At the time this book was written, our database contained about 120,000 entries. Initially, we used paper questionnaires, but now we administer it over the internet. You can answer this questionnaire at the following address: www.theWep.org (choose “Chronotype Study” once you are past the introductory page). If you enter your email address you will receive a .pdf file describing your chronotype in relation to others. It’s best to go to the website before you read too much of this book because naive subjects answer questions more reliably.

  7.Originally the science of epidemics was a branch of medicine that dealt with the study of the causes and distribution of disease in populations. Epidemiology has been extended to include the study of normal conditions of humans, such as sleep, eating behavior, and so on. This extension is a logical consequence of the insight that a disease is, in most cases, an extreme form of a normally distributed human quality.

  8.To establish the distribution of a quality or a trait, a researcher decides on categories or “bins” with defined limits. In the case at hand, the distribution is based on bins of thirty minutes. Such a bin collects, for example, all midsleep times between 1:30 and 2:00 A.M., excluding 2:00 itself because it already forms the lowest limit of the next bin. The counts in each bin are then graphed, commonly as vertical bars. To compare different distributions that relate to populations of different sizes, one converts the actual counts in each bin to the respective percentage of the entire number of people investigated.

  9.Distributions that are completely symmetrical are called normal distributions or bell-shaped distributions (since they look like the cut through a bell); they are also sometimes called Gaussian distributions after the mathematician Carl Friedrich Gauss, who introduced them. If you record every day how long it takes you to get to work, you can calculate the average time of your commute. Of course the times will be slightly different every day. If the chances of being shorter or longer than average were not biased in any way, you would get a normal distribution with a perfectly symmetrical bell shape. You would, of course, always like to get to your workplace in the shortest possible time, and the forces acting against that are not unbiased; they will rather actively slow you down than actively speed you up. If you are slowed down, the hold-ups can be quite substantial in some cases. If the bias against reaching the mean of a distribution is more toward one side than the other, it will produce a slightly unbalanced distribution, similar to the one in our example. The chances of being a later chronotype than the population’s average are slightly higher than those of being earlier than the average.

  2. Of Early Birds and Long Sleepers

  1.To my mind, the brilliance of Baba Shah’s witty statement lies in ridiculing all attempts to move early risers closer to God than other chronotypes and in recognizing that different species occupy different temporal niches within the twenty-four-hour day.

  2.Some people think that this proverb refers to golden teeth as a sign of wealth, but in this case, “mouth” merely stands for an opening, equivalent to an open hand.

  3.Noon stays constant throughout the year except for some wiggling that amounts to no longer than a minute.

  4.For example, see Ian R. Bartky, One Time Fits All (Palo Alto: Stanford University Press, 2007).

  5.The period length of the lunar cycle is also caused by interactions of sun, earth, and moon.

  6.Seasonal changes exist even at the equator (for example, in the amount of rainfall), although day length stays constant at twelve hours throughout the year.

  7.This is especially true in German: “Frühaufsteher” and “Langschläfer,” early risers and long sleepers.

  3. Counting Sheep

  1.Modafinil is one of the drugs that make people feel quite normal in spite of not having slept enough. Caution: we still know too little about sleep, so we cannot predict the long-term consequences of suppressing the effects of sleep deprivation. The state we are in when having slept too little is, like pain, part of a warning system that we most probably should not ignore.

  2.An oscillator is anything that can produce a rhythm: mechanical oscillators such as a pendulum or a swing; electrical oscillators such as doorbells; chemical oscillators that produce rhythmic transitions from one chemical state to another and back (often visualized by changing colors). The biological clock is also an oscillator.

  3.S. Daan, D. G. Beersma, and A. A. Borbely (1984). Timing of human sleep: Recovery process gated by a circadian pacemaker. American Journal of Physiology 246:R161–183.

  4.Sleep researchers call this the “wake maintenance zone.”

  4. A Curious Astronomer

  1.As Serge Daan recently pointed out in a lecture on the history of the field, there is no evidence that de Mairan used a desk or cupboard to perform his experiments. This notion probably goes back to one of the first textbooks on biological clocks, written by M. C. Moore-Ede, F. M. Sulzman, and C. A. Fuller: The Clocks That Time Us: Physiology of the Circadian Timing System (Cambridge, MA: Harvard University Press, 1982). In this book, the story of de Mairan was illustrated by a drawing showing a mimosa plant placed in a desk.

  5. The Lost Days

  1.Some of these bodily rhythms included: the alternation between sleep and wakefulness and between rest and activity; the ups and downs of body temperature, hormones, electrolytes, and cognitive performance; and how fast or slow subjective time passed.

  2.A Latin-derived construction for “about one day.”

  3.Notably also the acronym for rapid eye movement, which is the term for the stage in our sleep when we often have dreams, move our eyes, and twitch with our muscles as if we were awake.


  4.For example, subjects are shown a light for a certain time (or hear a tone) and have to say how long it lasted; in other tests they have to press a button and reproduce the length of the signal they just saw (or heard), or they are told to produce a given time by pressing a button. In the bunker experiments, subjects were merely asked to press a button when they thought an hour had passed and press the button again when they thought a minute had passed.

  5.The rules of the theory of evolution are simple. An organism’s genetic code, its deoxyribonucleic acid, or DNA, holds the information about every biochemical “tool” that cells need for living. Every offspring (of either sexual or asexual reproduction) can accumulate random changes (mutations) in its genetic code. The number of copies of a given genetic code within a population depends on how many offspring inherit this code over a long line of generations. In this “game,” individuals compete with other members of their species. Those that pass on more copies to the next generation than others win the game. This competition is under constant pressure: from members of its own species; from other organisms (both enemies and food); and finally from the physical environment, such as heat, drought, or anything else living beings have to cope with. If a random change in the genetic code helps an individual to better cope with these pressures, its chances to proliferate its genetic code become greater; if the change decreases its coping abilities, its chances of proliferation decrease, and that particular genetic code will be eventually outnumbered by others.