Internal Time
Internal Time
Chronotypes, Social Jet Lag, and Why You’re So Tired
Till Roenneberg
Harvard University Press • Cambridge, Massachusetts • London, England • 2012
For Casper, Pauline, and Flora
Contents
Introduction
1. Worlds Apart
2. Of Early Birds and Long Sleepers
3. Counting Sheep
4. A Curious Astronomer
5. The Lost Days
6. The Periodic Shift Worker
7. The Fast Hamster
8. Dawn at the Gym
9. The Elusive Transcript
10. Temporal Ecology
11. Wait until Dark
12. The End of Adolescence
13. What a Waste of Time!
14. Days on Other Planets
15. When Will My Organs Arrive?
16. The Scissors of Sleep
17. Early Socialists, Late Capitalists
18. Constant Twilight
19. From Frankfurt to Morocco and Back
20. Light at Night
21. Partnership Timing
22. A Clock for All Seasons
23. Professional Selection
24. The Nocturnal Bottleneck
Notes
Acknowledgments
Index
Introduction
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. The body clock is not a new invention in the long line of evolution. You share your ability to internally keep track of the time of day with practically every other creature on earth, from other mammals right down to those organisms that exist only as a single cell. This means that an internal biological clock must be extremely important for life on this planet. Living without or against the biological clock would mean premature death by predation or starvation for most animals in their natural environment. As I will elaborate in this book, living against our biological clock also jeopardizes our health and well-being. In modern society, we rarely live in synchrony with our body clock. Some of us travel fast across many time zones, and others (about 20 percent of the working population in industrialized societies) have to work in shifts. If you have ever suffered from jet lag, you know how strongly we are affected when our body clock is out of synch with social time. But even if you don’t work in shifts and never travel across time zones by airplane, you can still suffer from a chronic type of jet lag, which we call social jet lag.
A book about clocks is evidently also about time, yet the time of the body clock is not necessarily the same as the social time that we use in everyday life to be on time for work, travel, appointments, or the evening news. Social time is the time people live by. In the nineteenth century, social time was local sun time: noon was when the sun had reached its highest point. This rather sensible long-standing convention about time was eventually challenged by trains: they could transport humans over long distances within a few hours, rendering local sun time impractical because passengers had to reset their clocks at almost every station. States and countries, therefore, adopted a universal system in 1884, subdividing the world into twenty-four time zones that all refer to a zero line, the longitudinal meridian that crosses the observatory in Greenwich, near London. Theoretically, social time could adopt any time frame, as long as everyone used the same one. The entire territory of mainland China, for example, is covered by a single time zone based on Beijing time. This book will tell you how these different time systems interact: sun time, social time, and your very individual internal time.
Your internal time is produced by your own body clock. It varies from individual to individual just as body height, eye color, or personality varies, and it interacts with sun time and social time. In spite of internal time being probably the most important to our health and well-being—more important than sun time and certainly more important than social time—it has been thoroughly neglected. Every day, we are awake for approximately sixteen hours until we render the control of our movements, thoughts, and desires to a state resembling unconsciousness, which we call sleep. These daily changes are so utterly obvious that the underlying biological mechanism remained unexplored for centuries. In synchrony with the rise and fall of the sun, animals awaken and sleep, plants open and close their blossoms, and plankton travel up and down the water column. All these rhythms are controlled by a biological clock that represents the twenty-four-hour day of our planet. The alterations between sleep and wake are not simply two states of our existence that are being flipped like a coin once a day. They are reflections of a continuous change in all our bodily functions; a change that involves both turning genes on and off and continuously changing the cocktail of hormones and transmitters in all our tissues.
I have been studying the biological mechanism underlying our body clock for decades in very different creatures ranging from single cells and simple fungi to humans. These studies were conducted either in the laboratory, where we try to control all environmental factors (such as light, temperature, and food), or in the real world—for example, in factories, where we measure different variables over the course of the day, or in the world at large, by simply asking normal people when they do what. My initial reason for studying the body clock was autobiographical and almost by chance. One of the pioneers of the field, Jürgen Aschoff, was the director of a research institute in the Bavarian countryside. Jürgen and his wife, Hilde, had six children, who attended the same school that I did. I became friends with all of them, despite age differences. The Aschoff family lived in a beautiful castle (“the Schloss”) on the slope of a hill in Andechs, a village near the Ammersee, one of the larger Bavarian lakes. Andechs was a long way out in the countryside with almost no commuting possibilities. The Aschoff parents, therefore, allowed their children to invite friends to stay whenever they wanted, even overnight. To be part of a large crowd of youngsters who were all extremely interesting and interested was a lot of fun, and I stayed at the Schloss as often as I could. I also got on extremely well with the professor himself and became more and more interested in the science that he and his colleagues pursued.
When I was about seventeen, I began to work as a part-time assistant at Aschoff’s institute during most of my school holidays. It allowed me to combine the fun of learning science, spending time with fascinating people, and earning some money—a dream situation. There were always many visitors: friends of the parents; friends of the children; but also lots of scientists, some of them internationally famous, who seemed to be engaged in endless scientific discussions. I had always loved science, but the atmosphere in Andechs gradually turned me into a junkie—this was exactly the life I wanted to lead.
Although I probably had assimilated more knowledge about body clocks by the time I entered university than most students do by the time they graduate, I started to study physics, in my view the most fundamental science of all. But I soon realized that the true aim of my interests concerned humans and that physics just didn’t get me any closer to understanding them. So I switched to medicine. But once again, I felt that this discipline would not be the right vehicle for my curiosity. Although I wanted to know everything about humans, the focus of my interests wasn’t on helping or curing them. I began to appreciate that I could only start to learn something about humans if I understood more about evolution, genetics, biochemistry, comparative physiology, and ecology, but none of these subjects was taught in any depth in medical school.
I finally ended up
studying biology. I enjoyed discussions with other students and scientists much more than attending lectures, and I read lots of papers and books that covered many more issues than those taught in a degree program. I felt that my learning only began when I started working in the lab, collecting experimental data and then trying to make sense of them. The attempts at making sense of data always were my utmost joy in science—and still are. I think that this joy, and the ways and methods I use to approach the mysterious world of data, go back to my intensive interactions with Jürgen Aschoff when I was at a very impressionable age.
After a detour into photobiology, neurophysiology, and brain research that lasted for many years, I finally returned to the field that investigates biological clocks (chronobiology) as a postdoctoral fellow. I spent my first postdoc time back in Andechs, working with Jürgen Aschoff—not as a student but as a (somewhat) fledged scientist. I remained a colleague and friend of “the old one,” as his family and his close friends called him (although he never ceased to be my mentor), until he died in October 1998. After two years studying annual rhythms in humans with Aschoff, I was eager to learn more about how biological clocks work in cells, how they generate an internal day with the help of molecules. So I decided to work with another pioneer of chronobiology, Woody Hastings, a Harvard professor. I was part of his team in Cambridge, Massachusetts, for almost four years and kept going back during the summer for many more years. Returning to Germany, I found a rocky academic landscape for someone who simultaneously studied humans and single-cell algae, someone who was more interested in the investigation of concepts, such as the biological clock, than in staying within the boundaries of the little boxes created by our disciplines and faculties. Where did I belong according to orderly German academic criteria—botany, zoology, ecology, anthropology, or medicine? I ended up in the medical faculty, specifically in the department of medical psychology. Ernst Pöppel, the chair of the department, provided this scientific home, where I still reside. He was one of the few also interested in concepts (especially those regarding time), and less focused on the specific model organisms that were used to study them.
Over the years of studying the biological clock, I began to realize that what clock researchers were discovering had an enormous significance for our everyday lives. I noticed that people were fascinated and eager to learn about the science behind circumstances of their daily existence, some of which they had never considered before. Once enlightened, they started to understand themselves (and others) much better, began to appreciate their own individual time, and were suddenly relieved of the weight of prejudice ridiculing their temporal habits: for example, being called lazy if you don’t wake up fresh as a daisy by seven o’clock in the morning; or being called a boring person only because you don’t enjoy going out with friends after ten at night.
In this book I tell a story about internal time, or rather many little stories covering different aspects of our body clock. Each of the twenty-four chapters—I’d be telling lies if I told you I chose to write twenty-four chapters by chance—has two sections, the case and the background. In the case, I lay out a story about internal time that the later background information explicates. In many of these cases I will manipulate the facts for the sake of a good story, but in all, the data concerning chronobiology are accurate. For example, no one knows exactly what passed through a certain eighteenth-century scientist’s head when he came terribly close to discovering the phenomenon of internal time (which then lay idle for almost a quarter of a millennium), but, based on what he wrote, I use my imagination to get the facts across to you. Some cases describe relatively recent discoveries made by contemporary scientists. They are written to draw your attention to a question and to help you imagine how a discovery could have taken place. Although the scientific facts of the discovery itself are historically correct, other details, such as names and places, may be purely fictional.
With the case stories, I want to raise your curiosity and pique your urge to reason. If you are puzzled by a case, first try to identify what you don’t understand, and then consider what parts of the story are reflected in your own life. The second section of each chapter describes the facts underlying the case story in detail. It should answer most of your questions and may help you relate what you read to your own temporal life.
The use of cases is part of the philosophy of problem-based learning. Its aim is to focus the mind on a problem without employing jargon or excessive scientific explanation. Problem-based learning is often applied in university education, particularly in medical, law, or business schools. Students, often working in groups, are asked to identify all the facts in the case and then work out the background behind the story with the help of textbooks, the internet, lectures, and specialists. The best part of problem-based learning is being forced to confront an everyday problem that is completely puzzling, at least at first. The drawback of traditional learning has always been the dissociation between the theory and its application. “Why do we have to learn this?” is probably one of the most frequent and justified questions teachers hear.
To understand the biology of the body clock does require some knowledge of biology. I have done my best to keep the biological explanations accessible for everyone. More detailed explanations can be found in the endnotes. Whether you are interested in just the case studies, the background information, or the more detailed scientific explanations in the endnotes, I hope you will gain a thorough understanding of internal time, your time. I want you to appreciate how important this concept is for living well. I hope you have as much fun reading this book as I had writing it—fun is the best way to understanding and allows us to remember new information without too much effort.
1
Worlds Apart
Ann woke to a hard and persistent knock on her bedroom door. After staying in bed as long as possible, she wrapped herself into a thick bathrobe, put on warm socks, and stumbled to the bathroom to brush her teeth. She didn’t say “good morning” to her father and didn’t expect any form of greeting from him, either. If she hadn’t pushed him grumpily away from the basin to reach the faucet, one would have thought that neither was aware of the other. It was a school morning, shortly before the Christmas break. Ann was, as usual, far behind schedule and, like her father, Jim, began the new day in a state of semi-consciousness. Before Jim had school-aged children he used soap and a razor, which he thought produced a much cleaner shave, but now that he was forced into this early routine, he tolerated the noise of an electric razor, having cut himself too often. His wife, Helen, was already downstairs making breakfast together with their son, Toby.
In contrast to father and daughter, who went about their morning routines in silence, Toby and his mother were chatting with each other like a pair of canaries. Toby was telling her about his field trip to the dinosaur exhibit and got quite carried away when enumerating the different raptors he had seen. While his mother was preparing sandwiches for the children’s school breaks, Toby set the table, but soon got distracted by the back of the cereal box, which he had just placed in front of his bowl. He read all about the new dinosaur collection that would be coming out next year—one in each package. He decided to eat more cornflakes in the future, at least two bowls every morning.
Helen always put extra effort into the sandwiches for her firstborn. She wanted to make sure that her daughter would eat them because she usually left the house with nothing more than a cup of tea in her stomach. By the time Ann had crossed the threshold into puberty, she had stopped eating anything before leaving the house, and Helen had fought—and lost—endless battles about “proper breakfasts.” It was actually Jim who had put an end to this struggle: “Make her a good sandwich with her favorite stuff on it and she will eat it in school as soon as she is hungry.” Of course, no parent can ever be sure what really happens to lunches from home, but the fact that Ann uttered requests from time to time for variations in the narrow theme of her favorite foods encouraged Helen.
At around seven o’clock Jim joined the talkative pair, kissed Toby on his way through the kitchen and then Helen, who handed him a big mug of coffee. The three sat down at the breakfast table, and Helen, as usual, yelled, “Hurry up, Ann, the bus will be here in twenty minutes!” They were lucky because the bus stop was right in front of their house, and Ann made the best of this short distance by coming downstairs only a couple of minutes before it arrived. Helen’s primordial cry was a remnant of her “proper breakfasts” fight.
At last, Ann did join the others, slowly sitting down at the table to sip her tea. Toby continued his lively dinosaurial narrative, mainly addressing his mother. He only approached his sister at breakfast if he was in a mischievous mood. She was an easy victim, unable to muster resistance, although later in the day she usually did get her revenge. Helen was half listening to Toby and half concentrating on planning the day, making her to-do list, and giving Jim or Ann short instructions. The chances that Helen could have a real conversation with her husband in the morning were greater in summer, when daylight flooded the kitchen and they occasionally had breakfast on the terrace. Now that the sun came up after the children were in school, all of them were more subdued than usual—even Toby and his mother. Helen was thinking of the PTA meeting scheduled for that evening and decided to ask Jim to go—he was much more up to it at that time of day, and she could go to bed early.
Ann brooded about the school day ahead of her. Why math in the first period? Why not art or history? She was actually quite good at math, but she needed at least half a functioning brain to solve mathematical problems, and most of her brain certainly didn’t wake up before ten o’clock or even later, no matter how early or late she got up. When Ann left the room to get her coat, Jim was able to produce the first smile of the day when he read the back of his daughter’s tee shirt: “Early to rise and early to bed makes a bird healthy, wealthy, and dead.”1
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