This first case represents—with minor variations—the morning routine of millions of households across the globe. In that sense, it is almost trivial. But apparent trivialities will play an important role in this book. How easily do we wake up in the morning, and why? Jim, Helen, Ann, and Toby seem to be worlds apart at this early hour: Helen and Toby feel fresh, and Jim and Ann feel woolly. The case is rich in information that you might have absorbed without even noticing, but it also triggers many questions: Is this a sex or gender issue?2 Does age play a role? Does the ability to wake up depend on the time of day? Are different wake-up types also different fall-asleep types? What do eating habits have to do with these different wake-up types? Does performance in such different activities as math and art depend on the time of day? The wake-up type? Some of these questions can be answered by the story itself.
Jim, Helen, Ann, and Toby’s morning touches upon many different aspects of temporal life and biological clocks. We are told when the story takes place (an early weekday morning shortly before Christmas), but we need to know where the family lives. Our conclusions will differ radically if the family lives in South Africa, Peru, or Australia rather than Europe, Japan, or the United States. But the story gives several hints to indicate that the family lives somewhere in North America or northern Europe: if it is nearly Christmas and dark outside, the family must live in the northern hemisphere. The size of the house and family and what they eat may provide other hints.
It would also be helpful to know the approximate ages of the family members. Let’s start with an educated guess: Toby appears to be younger than Ann, and Jim is probably older than Helen. Toby must be about six or seven because he is still young enough to be fascinated by dinosaurs but old enough to read the back of the cereal box. Ann, firstborn, has entered puberty but still lives at home, so let’s put her at fourteen.3 Helen and Jim, their parents, therefore might be in their early forties. Theoretically, Helen could be much younger and Jim much older, but that is somewhat beside the point.
Now let’s turn to the central theme—the different wake-up types. Father and daughter are not exactly communicative. Is Ann mad at her father or does her grumpy behavior merely reflect her resentment of having to get up at that ungodly hour? We could ask the same questions about Jim. By contrast, Helen and Toby are in the best of moods, already fully awake and active. Is this just the normal contrast between teenagers and children, or could these behavioral differences be—at least partly—accounted for by different wake-up types? Being a certain wake-up type is obviously not a simple matter of age, sex, or gender since both Helen and Toby are more awake than either Jim or Ann.
Once again, the story answers our questions. Before Jim had children at school, he felt vigilant enough to give himself a clean shave. Ann is apparently quite good at math after ten o’clock. Different wake-up types are apparently also different fall-asleep types. Helen is fresh in the morning but feels sleepy quite early in the evening. Although Jim has to get up on weekdays at approximately the same time as Helen does, he is dead tired in the morning but still quite alert in the evening—the PTA meeting will, therefore, be his task.
We know from experience—starting in our own families—that individuals possess different timing types, or chronotypes.4 In many cultures and languages, chronotypes are often named after birds—early birds and late birds.5 The common usage of larks and owls suggests that we are dealing with two categories. A Danish researcher has recently coined them, less poetically, A and B types, supporting the notion of two categories. However, the attempt to categorize any population of living beings into two categories is rarely correct. In general, human qualities, including chronotype, almost never fall strictly into two simple categories.
My colleagues and I have investigated human chronotypes for many years by asking thousands of people about their sleep habits with the help of a questionnaire.6 We use the answers to these questions to define a person’s chronotype. Defining the timing of an event is not necessarily straightforward. “When did you hear the shot?”; “When is high tide?”; or “When did the sun rise?” are easy questions because they concern clearly definable events. “When do you usually sleep?” is more complicated because we usually sleep for seven to eight hours. We therefore ask “When do you usually fall asleep?” or “When do you usually wake up?” But the answers to even these questions are difficult to translate into a person’s chronotype. Let’s suppose that Person A sleeps from 10 P.M. to 6 A.M., Person B from 10 P.M. to 8 A.M., and Person C from midnight to 6 A.M. If chronotype were defined by sleep onset, A and B would be the same type. If one defined it by sleep end, A and C would fall into the same category. The difficulty arises because sleep has (at least) two different and independent qualities: sleep timing and sleep duration.
It turns out that the midpoint of sleep is best for defining a person’s chronotype and also solves the problems described above. The calculation of midsleep is easy: if a person usually falls asleep at midnight and usually wakes up at eight, then his usual midsleep is 4 A.M. All these sleep times should represent what is done daily, not what is the exception, such as a party or a late night at work. Midsleep of Person A would be 2 A.M., that of Person B would be an hour later at 3 A.M., and Person C would have the same midsleep as B but would sleep four fewer hours, going to bed two hours later and waking up two hours earlier.
Our large database allows us to investigate the epidemiology of sleep behavior in different populations worldwide.7 The figure shows the distribution of midsleep in Central Europe (containing the answers from approximately 100,000 participants, predominantly Germans).8 The distribution is almost a perfect bell shape, although late types are slightly more numerous than early types.9 Categories like larks and owls, A and B people, misrepresent the continuous distribution of chronotypes as much as dwarves and giants misrepresent the distribution of body height. These opposites simply label the extreme types at both ends of distributions, which are extremely rare.
We base the first assessment of an individual’s chronotype on her sleep behavior on free days, when it is not dominated by work or school times but rather by individual preference, by a body clock that dictates her internal time. Slightly over 14 percent of the population (represented by our database) fall into a midsleep category of 4:30 to 5:00 A.M. Presuming a sleep duration of eight hours (to make things simple), individuals in this category go to bed on free days between half past midnight and 1:00 and wake up between 8:30 and 9:00 A.M. The midsleep times (on free days) of over 60 percent of the population fall between 3:30 and 5:30 A.M., but only less than half a percent between 1:30 and 2:00. These extreme larks begin their sleep on free days between 9:30 P.M. and 10:00 and wake up voluntarily between 5:30 A.M. and 6:00 (again assuming an eight-hour sleep duration). There are even more extreme early chronotypes, but their number is so low that the corresponding vertical bars are too small to be detectable in this graph. On the late side of the chronotype distribution, about four percent fall asleep between 3:00 and 3:30 A.M., and many more people sleep even later.
The distribution of midsleep in Central Europe.
So far, we have identified chronotypes primarily based on sleep habits, but it will become clear throughout this book that chronotype means much more. The internal timing of our body clock, of which sleep habits are just one aspect, dominates all functions in our body, ranging from genes being activated at certain times, to changes in body temperature and hormonal cocktail, right up to cognitive functions like the ability to do math. Magazine articles about the body clock often tell their readers when to do what: when to go to the dentist because the pain sensation is minimal; when to exercise because the training effect is greatest; when the best times are to do math or write poems or even when to make love. I am amazed and happy that the media pick up on the importance of our biological clock but also a bit worried that many of these articles approach this phenomenon naively. Some of them advise their readers, for example, to have sex in the morning. This advic
e is merely based on the fact that the male sex hormone testosterone peaks in the early morning hours. This may lead to an increased sex urge in men and may even ensure better male “performance.” But does this necessarily mean that sex is optimal for both partners at this time of day? Our biology concerns all aspects of our being, including our psychology and feelings, from pain to pleasure, and all these facets put together are more complex than the concentration of a single hormone, however important testosterone may be for sex. Thus, the media should use care when giving advice. This holds especially in light of what we have learned in this chapter: different people can have very different chronotypes with the extremes being up to twelve hours apart. So even if a piece of advice about when to go to the dentist was valid and useful, it would only be of limited help because an individual’s internal time could theoretically be as many as six hours earlier or later than the advised time.
We are capable of different things at different times of the day. Many aspects of performance show daily fluctuations, and their highs and lows depend on chronotype. Not only performance but many other aspects of life vary according to the time of day. If we wake up in the middle of the night, we don’t cook ourselves a meal—we are not hungry. So why should Ann be hungry just because she is being woken every school day at around her internal midnight? The quotation on Ann’s tee shirt questions why the early hours of the day have such a good reputation, with all those proverbs glorifying the early risers. Good question!
2
Of Early Birds and Long Sleepers
The sun had just risen above the horizon when the farmer walked along a country path toward his field. He cheerfully greeted a man he encountered halfway between the village and his destination. He thought to himself, “Must be a decent bloke to be up so early.”
The postman rang the doorbell at half past ten. If he hadn’t heard noises from within the apartment, he would have left long before. He rang the bell once more—longer and harder—and then heard a rather grumpy voice announcing the imminent appearance of its owner. “Lazy bugger,” thought the postman when the young man appeared in a robe and with sleep-tousled hair.
The journalist had waited for quite some time on the phone. She was finally put through to the scientist. “Thank you very much, professor, for making the time to answer my questions. I wanted to interview you about early birds and long sleepers.” The researcher looked toward the heavens and suppressed a long sigh, knowing that this interview would involve debunking many myths and explaining many basic principles.
The moral backbone of practically every culture declares early risers as good and late risers as bad people. God rewards the early risers; they are more effective (“an hour in the morning is worth two in the evening”) and will be wealthier than the “lazy” rest of the population. The larks are declared the successful and productive members of society while owls are, at best, extroverted artists and intellectuals, or at worst, people who engage in dark arts and exert evil powers. Folk wisdom praising early risers can be found in many places around the globe: “The early bird catches the worm,” “Zoqde nio er yu chóng chī” (United States and China); “Morgenstund hat Gold im Mund,” “De ochtendstond heeft goud in de mond” (“The morning hour has gold in its mouth”—Germany and the Netherlands); “À qui se lève matin, Dieu aide et prête la main” (“God helps those who rise in the morning and gives them a hand”—France); “Chi dorme non piglia pesci” (“Those who sleep won’t catch fish”—Italy); “A quien madruga Dios le ayuda” (“God helps those who get up early”—Spain); “Kto poran’she vstaët, tot gribki sebe berët; a sonlivyi i lenivyi idut posle za krapivoi” (“The early riser gathers mushrooms, the sleepy and lazy one goes later for the nettles”—Russia).
Many of them are mere translations or variations of the early-bird-catches-the-worm story, replacing the bird with a human and the worm with mushrooms or fish. But there are other stories, like the one I received from Sato Honma from Sapporo, Japan:
Dear Till,
“Hayaoki wa san-mon no toku” means “early risers earn 3 mon” (3 mon is about 1 dollar). The origin of this proverb is not clear, but an interesting origin is as follows: Maybe 200–500 years ago, deer were highly treasured in Nara. If a deer was found dead on the property of Nara residents, they had to pay 3 mon. So, the proverb says that if you rise early and you find a dead deer in your garden, then you still have the chance to move it to your next-door neighbor’s property.
Sato
My learned friend Amitabh Joshi from Bangalore surprised me with his statement that such proverbs apparently do not exist in Hindi or Urdu:
Dear Till,
Although waking up early has traditionally been considered “good,” there is no saying to this regard, to the best of my knowledge, in either Hindi or Urdu.
There is, however, a couplet from a Punjabi Sufi poet, Baba Bulley Shah, to the effect that if God could be attained by getting up early, surely the roosters would have found God.
Cheers,
Amitabh1
Early-morning morals make perfect sense in cultures where the entire population shares common rest and activity times and where productivity is predominantly restricted to those hours of the day that are lit up by the sun. In those cultures, the availability of resources has not only spatial aspects (where to find the food) but also a temporal aspect (when to find the food). Time of day has been of ecological importance ever since different individuals and organisms started to compete with each other. Early-riser morals probably reflect a biological truth that actually formed the evolutionary basis for why the family members in the preceding chapter are (time-)worlds apart.
The Russian version of the early-riser wisdom is an excellent example of how temporal ecology in human populations leads to a temporal economy. “The early riser gathers mushrooms, the sleepy and lazy one goes later for the nettles.” Thus, if the early birds gather all of the mushrooms before competitors appear on the scene, they have enough mushrooms to feed their own families and a surplus to sell on the market—a twofold economic advantage. The early riser gets the food for free and can even sell the food to the sleepy and lazy ones—thus, “the morning hour has gold in its mouth.”2 A modern interpretation of the proverb would be “Larks are rich” or “If you want to be rich, be a lark.”
The twenty-four-hour day is a circular event and, as such, represents a temporal structure. Generally, time has no structures except for those we have created artificially, such as minutes within an hour. Unlike the temporal structure of a twenty-four-hour day, minutes and hours have no concrete beginning or end—they are mere units on a stopwatch or timer. The temporal structure of a twenty-four-hour day is tightly linked to the rotation of the earth and the consequential periodic exposure of its surface to the sun. It has a beginning and an end—the end coinciding with the beginning of the next day. It doesn’t really matter where we set the beginning of the day in relation to sun time. Letting the day begin with dawn makes sense since this more or less coincides with the onset of human activity. It has, however, the disadvantage that the time of dawn changes with the season (except for locations on the equator), earlier in summer and later in winter. To avoid this problem one could let the day begin at noon, which is constant throughout the year.3 But then, midday isn’t really the best choice for the beginning of the day. That is why we have chosen its counterpart, midnight, as the zero hour. There are excellent books that describe the history of how humans have dealt with time and how they have organized time, so I will not go into more detail here.4
The twenty-four-hour day is not the only time structure on earth—there are three others. The tides—caused by the interactions between moon, earth, and sun—concern all those organisms living on the ocean’s coastlines. The tidal peaks and troughs recur every 12.5 hours. The period length of the lunar cycle is 28.5 days, and the duration of a year is 365.25 days.5 One might think that years were counting days, but that is not the case since the year does not depend on
the earth’s rotation around itself but on its orbit around the sun. The fact that this orbit takes a quarter-day longer than 365 days is the reason for introducing leap years. Theoretically, our planet’s spinning around its own axis could produce different day lengths without affecting the length of a year. As a matter of fact, that has been the case throughout earth’s history; many millions of years ago days were shorter than they are today by several hours. The spinning of the earth is slowing down, so our days will be longer than twenty-four hours far in the future.
The temporal structures of tide, day, month, and year deeply affect the earth’s environment and therefore also have a strong influence on all forms of life. But not all organisms are exposed to all four temporal structures. Except for some cave-dwelling organisms or those living at great depth in the ocean, practically all beings are exposed to the endless repetition of twenty-four-hour days as well as to the seasons of the year.6 Tides are only important for organisms that live in tidal zones or for those that feed on tidal organisms. Finally, there are relatively few organisms for which the waxing and waning of the moon’s light forms an essential part of their existence.
When organisms are exposed to a regularly changing environment, it is advantageous for them to adapt to these temporal structures, to be prepared for and to anticipate the regular changes. Temporal structures provide the only context in which we can actually predict the future. If I were to bet a large sum of money that a certain series of numbers would be drawn in next week’s lottery, you would immediately bet against my prediction. If, however, I bet you that the sun will rise tomorrow, you would just laugh at me. You would certainly not bet against my prediction. The predictive power within temporal structures is an advantage that drove the evolution of biological clocks.
Internal Time: Chronotypes, Social Jet Lag, and Why You’re So Tired Page 2