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

by Roenneberg, Till

  De Mairan had a hard time concentrating on his manuscript that day. It was again beautiful, and he decided to work outside so that he could keep the curtains in his study drawn. Every hour or so, he went inside and peeked into the darkness of his desk. The leaflets stayed in a horizontal position throughout the entire day, started to fold downward in the late afternoon, and were fully collapsed even before the sun had completely set. When he went to bed at around midnight, the mimosa was fully “asleep.”

  Over the course of the next few days, de Mairan remained so excited by his observations that he slept very badly. After almost a week of severe sleep deprivation, he was so tired that he fell into a deep sleep sometime between 9 and 10 P.M. He woke up once during the night, looked out of his bedroom window, and saw that, though it was still dark, dawn was beginning to break. He went downstairs to have another peek into his desk. His study, with drawn curtains, was still pitch black and he had to light a candle to find his way. He put it down as far away from the desk as possible. Being adapted to the night’s darkness, he was able to see very well in the barely lit room. He opened his desk’s door a tiny crack, enough to see that the mimosa had already started to lift its leaves. He continued his little experiment for several days and found that the mimosa’s leaf movements continued with the same regularity as if it were still standing on the windowsill.

  The plant evidently continued to “know” the sun’s position and continued to be “aware” of night and day. During the later stages of his experiment, de Mairan tried to keep an early schedule so he would be able to open the door not only after the sun had set but also before any sunlight could enter his study through the tiny slits between the curtains. Every morning before the sun had properly risen, the leaves were up, and every evening just before the sun had set, they were furled. The mimosa was like a sick person who lies in bed in semidarkness for days but still sleeps during the night and is more or less awake during the day without ever seeing true daylight. He concluded that leaf movements, at least those of the mimosa, were not merely a reaction to light and darkness.

  Throughout the remainder of the summer of 1729, de Mairan repeated his experiment many times. He allowed the plant to vegetate in constant darkness for only a couple of days at a time before he exposed it to the sun again, knowing its need for light. He even borrowed mimosa plants from friends to repeat the same experiment and always found that their leaves’ movements continued in constant darkness. He became so fascinated by his findings that he abandoned the manuscript he had sworn to finish by the end of the summer and, instead, wrote down his “botanical observations.” It would be interesting, he thought, to see whether other plants behaved the same way. It obviously wasn’t light that controlled the up and down of the leaflets—maybe it was temperature, which could easily be tested by using an oven. Even though the rhythm of leaves apparently came from within the plant, independent from the outside light, he wondered: could one still reverse the leaves’ rhythm by reversing the natural course of day and night with the help of artificial light?

  De Mairan’s botanical observations are the first traceable scientific source that mentions the independence of daily rhythms from the changes of night and day, of dark and light. His published accounts constitute one of the most concise scientific papers, consisting of fewer than 350 words and, as typical for the French writing style of the time (or even of now), of only seven rather long sentences. His observations were reported to the French Royal Academy of Sciences in the same year he performed the mimosa experiments by his distinguished colleague M. Marchant, a fellow of the Academy, and were subsequently published in the Academy’s proceedings. The paper’s last two sentences state that the daily occupations of M. Mairan (namely, astronomy) had prevented him from conducting any of the experiments proposed in the paper. He contents himself with a simple invitation to botanists and physicists to do so, even though they themselves may have other assignments to complete. But he warns them that the progress of true science is inherently slow.

  Everything in de Mairan’s paper turned out to be true, including his last statement about the speed of scientific progress. Although de Mairan’s observations were sporadically picked up by other botanists and zoologists (including Darwin), it took 200 years before botanists, at first, then zoologists, and then human physiologists started to investigate with modern techniques the mechanisms behind the enigmatic internal clock discovered by a French astronomer who was far too busy to pursue his observations with more experiments.1

  5

  The Lost Days

  The young man sat at his desk looking extremely pleased with himself. He was on schedule; in two weeks his thesis would be finished. He got up and walked through his windowless studio apartment to the small kitchenette to put on some water for a cup of coffee. While waiting for it to boil, he added a couple of items to a shopping list lying on the kitchen counter—coffee, milk, butter, . . . After checking his list once more, he opened the solid, soundproof door of his apartment and placed the piece of paper on one of the shelves in a small, dark walk-through cupboard outside of his apartment. The cupboard led to another door, now closed.

  On his way back into the apartment, which had served him as bedroom, study, dining room, and living room for many weeks, his expression changed—he had remembered something. He walked quickly to his desk where he pressed a button, as if he were ringing a bell for room service. Nothing happened, and about one minute later he pressed the button again. Throughout the day he had repeated this procedure fifteen times. Although nothing ever happened after pressing the button, he did not appear to be surprised. With a sharp whistle, the kettle begged to be taken off the stove. The young man turned swiftly and almost stumbled over the long cable leading from his belt to a plug in the wall. While he was pressing the button he had changed his mind about the coffee—he would instead make a bowl of instant soup. He had been writing all day and was tired.

  Once he had eaten his supper, he undressed, took a shower in his tiny bathroom, and climbed into bed, reaching for his diary on the bedside table. Conscientiously and meticulously he described all his activities, his physical state, and his feelings over the past sixteen hours. Then he got out of bed and pressed the button on his desk once more and, when he thought a minute had passed, one last time for today. He slipped between his sheets and buried his head in the pillows, without turning off the overhead lights covering most of the ceiling behind a layer of milky glass.

  While the young man was sleeping, a young woman carrying an empty tray went down a flight of steps leading into a small hill. It was a brilliant spring morning, and she had just arrived at work. After opening a thick and heavy door she entered a dark room filled to the ceiling with scientific equipment and cables. She switched on the lights, looked at the large plan of two apartments mounted to one side of the wall, and checked the different lamps and settings. Having entered all the collected information in a notebook, she turned and opened another door giving way to a dark walk-through cupboard lined with shelves. She retrieved some capped bottles and a small piece of paper. Placing everything on a tray, she walked out of the hill, closing all the doors behind her, and proceeded to a larger building. Once inside, she entered one of the rooms leading off a long corridor lined with wooden, built-in cupboards and placed the tray on the bench top filled with laboratory equipment.

  At the other end of the laboratory, two scientists were discussing data, and one of them turned to the young woman who had just entered the room.

  “How’s everything down in the bunker?”

  “He’s asleep, but his list says he needs a few groceries. As soon as I’ve analyzed the urine samples, I’ll pop down to the shops.”

  “No need for that, Karen, it’s his last day. In about ten hours, once he wakes up, we’ll go in. He’ll be in for a surprise.”

  “So I’d better go and get today’s newspaper,” Karen replied with a broad smile.

  Several hours later, the young man woke up. He fe
lt refreshed, and cherished the fact that, like every day during these long weeks, no alarm had woken him. Like yesterday evening, he got out of bed, went over to his desk, and pressed the button once and then again a minute later. To his great surprise, for the first time, his action prompted a response—the door opened and three scientists entered, an elderly professor and two coworkers.

  “What are you doing? Has anything gone wrong? Why are you terminating the experiment? Didn’t you say there would be no contact until the two months were over, or if I decided to quit?”

  “Yes, we did,” said the elderly professor, “and we aren’t breaking the rules. Congratulations and sincere thanks for a very successful experiment—your lonely weeks are over.”

  As if on cue, the young woman who had previously collected the urine samples produced a newspaper with a broad smile and handed it to the astonished young man, who looked at the front page and read the date.

  “This is a practical joke—there’s no way I can be wrong by two weeks. I knew that living without a clock would probably get me out of synch with the outside world, but . . . What time is it?”

  He asked this last question with great eagerness, realizing how little he had missed the usual temporal certainty during the past weeks, but how important it had suddenly become as soon as he noticed the wristwatches everyone was wearing.

  “It’s 8 P.M., the fourth of April. You’ve lived in the bunker for sixty-three days as one of our best subjects. While all of our days had exactly twenty-four hours, many of your days were as long as forty or even fifty hours.”

  “How can that be? I pressed this button twice every time I thought an hour had passed, and I never did that more than sixteen times a day. Does that mean I wasted most of my time sleeping? I thought I was keeping to such a strict schedule on my thesis,” he lamented.

  “No, as a matter of fact, you never slept more than a third of your days, just like in real life. When you were awake sixteen hours, you slept eight hours; and when you were active for thirty-two hours, you slept sixteen; but during those long days, the time span you thought to be an hour doubled, and so you never noticed the change.”

  “But I never had more than three meals per day. I never felt especially hungry, and I never ate any extra big meals.”

  “That is why we are so intrigued with this experiment. When you extended your days you truly expanded time: your hour estimation doubled, and you kept to your habitual meal times although that meant not eating anything for as long as sixteen hours. Despite this, according to our records, you haven’t lost weight. We don’t know yet how to explain all this, but the contacts on your apartment floor tell us that you were less active during those long days; maybe that begins to explain why you didn’t lose weight despite eating about half the amount of calories per twenty-four hours. The only function that didn’t go along with the extended days of wake and sleep was your body temperature. It continued through your long days with its usual twenty-five-hour day. So, while your sleep–wake cycle occasionally lived through forty-to fifty-hour days, other parts of your body kept to a circa-twenty-four-hour day.”

  The young man forgot his initial shock at having lost almost two weeks of his life and started to listen to the scientist’s description with growing interest.

  “During the first week of your experiment, when you still had a watch and when we kept the doors to the bunker open, you stayed synchronized to the normal twenty-four-hour day. You went to bed at around midnight, slept through your temperature minimum at approximately 4 A.M., and woke up after about eight hours of sleep. Then, we took all of your clocks away except for the one in your body, and you started to live your own days—your body clock began to ‘free-run.’ You gradually stayed up longer, approximately 1.5 hours every cycle. Your temperature rhythm also became longer, but only by about an hour. After a couple of cycles, you went to bed around the time when your temperature rhythm hit a trough.

  “For the next week, your activity–rest cycle and your temperature rhythm remained synchronized with each other, both producing days of approximately twenty-five hours, and you always went to sleep when your temperature was at its lowest point. This is what most subjects in the bunker do, but in your case, after about ten days into your clockless existence, your activity–rest cycle started to run at a slower pace than your body’s temperature rhythm—the two rhythms uncoupled. For many days, they appeared to take no notice of each other. Only when your sleep–wake rhythm was delayed so much that the time you went to bed came near your temperature minimum again did the two rhythms resynchronize for a couple of cycles before they broke loose again.”

  The young subject had millions of questions, but the professor suggested they walk up the mountain to the beer hall of the monastery to celebrate the end of a great experiment, and there they could answer his questions—if, he said, winking, they knew the answers.

  Botanists had returned seriously to the century-old question of how internal clocks work in the 1930s. They had proven in many experiments that de Mairan’s observations were real and had worked out many rules about the behavior of this biological clock under different light–dark or temperature cycles, predominantly in plants. However, biological clock research only received scientists’ full attention after the Second World War. The two main pioneers of the young research discipline were Jürgen Aschoff in Germany and Colin Pittendrigh, who worked at Princeton University at the time and later joined Stanford University. In the early 1960s, Aschoff was appointed one of the directors at one of the research institutes of the German Max Planck Society initiated by the founders of modern behavioral physiology, Erich von Holst and Konrad Lorenz. The newly created institute was situated in the heart of Bavaria near the “holy mountain,” one of the best-known traditional beer-brewing monasteries, Kloster Andechs.

  Aschoff and his colleague Rüdger Wever wanted to investigate whether humans were also governed by an internal timing system, by a body clock, as had been shown for many different plants and animals. Although most of the scientists who investigated biological clocks had no doubt about their existence, some researchers still believed that one could never create a completely time-free environment. They argued that the observed ongoing rhythms could still be controlled by some unknown factor linked to the rotation of the earth. Aschoff and Wever, therefore, set out to build the Andechs “bunker,”’ two small time-free apartments inside a hill. The interior of the bunker was shielded against everything that could disclose any time-of-day information to the subjects. It had no windows, and was completely soundproof and shielded against vibrations caused by the heaviest vehicles driving on nearby roads. It was even equipped with a metal cage keeping out the more-or-less regular changes of the earth’s electromagnetic field.

  The apartments could be entered only through a corridor separated by two thick doors, each of which could be opened only if the other one were closed. This hallway served as a link between the time-free world inside to the time-driven world outside. Subjects placed their shopping lists or bottles with urine samples on the shelves of this corridor. The urine was used to monitor the daily rhythms of metabolites, such as potassium or calcium, as well as substances that allowed the scientists to estimate ups and downs of hormones. The scientific staff looking after the bunker experiments and their subjects entered the corridor with high irregularity, even at odd hours during the night, so that the subjects could never deduce normal work hours by the times when their shopping lists and samples were collected, or when the requested items and new empty sample bottles were delivered. The floors of the apartments contained electrical contacts that were used to record when, and how much, the subjects moved. To record their body temperature, subjects had to wear a rectal probe, which—in that pretelemetric era—was connected by a long cable fixed on a belt around their waist to a wall socket. Subjects were asked to keep detailed diaries about how they felt both physically and psychologically.

  Depending on the scientific question, the duration of the
experiments varied between one and several weeks. Sometimes the apartments were separated into two completely independent units, each inhabited by a single subject; other experiments involved a group of people and investigated how the body clocks of different individuals influenced one another.

  Although free-running daily rhythms had been described extensively in plants and animals by the time the bunker experiments started, the results from putting humans in temporal isolation were almost eerie—even for those scientists who had hoped that the biological clock in humans would behave like that in other creatures. During the first days of each experiment, subjects remained in contact with the normal daily routine of the outside world, with regard to both staff and daylight. Once they were isolated from any time cues, their daily routine continued almost normally with its usual structure: two-thirds awake and one-third asleep. One of the important differences between the real world and life in the bunker was that the periodicity of the self-chosen bunker days wasn’t exactly twenty-four hours but in most subjects slightly longer. Another important difference was that subjects usually went to sleep at around the time when their body temperature hit a daily low point. This varies from what we do when we live in the real, time-driven world: our body temperature hits its low point about halfway into our night’s sleep.