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

by Roenneberg, Till

  A year later, Joseph married Sophie, whose parents owned a farm just twenty miles as the crow flies. Soon thereafter Frederic and Hanna also married and moved into the attic of the farm’s main house, which the twins had started to remodel soon after the first wedding. Hanna had always been more of a farm girl at heart, feeling slightly out of place in her parents’ grocery trade. They all could have led a normal, uncomplicated, and happy life had it not been for the evenings and the mornings.

  Until Frederic started to work in the factory, the twins lived as if controlled by the same clock. At the same times—almost down to the minute—they would start to yawn, to fall asleep, and to wake up (without alarm clocks). But the longer Frederic worked in the factory, the more the twins’ timing drifted apart. While the farmer kept to his old habits, the factory worker went to bed later and later and now definitely needed an alarm clock. Much as the young wives had in common, they were almost opposite in their daily habits. Hanna had to be in bed at around eight. She fell asleep immediately and was up well before the sun (except for a couple of weeks during the summer). But Sophie had always been a late type. In his speech at their wedding, her father had praised all her virtues but had warned Joseph how difficult it was to get her out of bed. So it came about that Joseph and Hanna took to tending to the cows in the morning and then making breakfast for the others. At the other end of the day, Frederic and Sophie would be the last to go to bed, long after their respective spouses.

  Common life stories and practical issues had helped in making the matches, but now the daily routine started to reshuffle the quartet’s emotions. Frederic and Sophie grew closer and closer during their shared evenings. Hanna and Joseph strengthened their bonds during the early morning hours. The changes occurred so gradually that none of them noticed. But one night Hanna woke up, went to the kitchen to make herself a warm cup of milk, and found her husband in the arms of her (and his) sister-in-law. Hurt feelings are the poison of harmony. The incident threw the quartet into a maelstrom of accusations and distrust, made worse by the fact that—over the next months—no one got enough sleep anymore. Hanna and Joseph stayed up well beyond their usual bedtimes so as not to leave their spouses unguarded in the evenings, and Sophie and Frederic made an effort to get up early because they too distrusted their partners.

  Hanna fell into a deep depression and stopped commuting into town with her husband to work in the family’s shop. At moments when she could think more clearly, she had to acknowledge that she had also started her early mornings joyfully, anticipating her time with Joseph. When his hand accidentally touched hers during work (which had happened more and more frequently), she drew hers back quickly, because the feelings these incidents triggered filled her with guilt. In the end, paradoxically, it was Hanna’s worsening depression that had saved them. She hardly left the house anymore and was starting to go to bed as early as six, getting up at two. The other three sat together one evening and Frederic told them about the difficulties Hanna’s aging parents were having in keeping the shop running without their daughter’s help. “If only I had more time,” Sophie said, “I would go and give them a hand.” Frederic suggested that Hanna could take over some of her responsibilities on the farm. “She would have to be outside much more, and that would surely also help in fighting her depression,” he said. The plan was put into action, and the new working constellations somehow seemed to relieve the emotional tension as well. In the end, the quartet found a solution that was ideal for everyone. From then on, they were a happy extended family once more, one that became even happier when the two couples became—over the years—parents of a considerable number of children.

  Although chronotype can influence a relationship, Hanna’s difficulties and Frederic’s new sleep schedule illustrate the importance of zeitgeber strength, highly relevant in our modern society. Joseph and Frederic are identical twins who grew up under identical conditions on their parents’ farm. So the genetic basis on which their body clocks are built is identical, and their daily behavior was originally completely synchronized.1 Their temporal habits drifted apart only when Frederic started to work in the factory—he appeared to become a later chronotype. This phenomenon can be explained by the principles of entrainment. Recall the hide-and-seek game of entrainment. To make the length of the internal day the same as that of the external day, the clock has to expose the appropriate portions of its response characteristics in the light and hide the other portions in the dark. I have provided an example of this hide-and-seek game with three theoretical body clocks—the twins’, Hanna’s, and Sophie’s—whose internal days are either exactly twenty-four hours, shorter, or longer. According to the rules of entrainment, each of these body clocks has to expose different portions of its response characteristics to the light (hiding the others in the dark). Individuals whose internal time is controlled by these clocks would each have a different chronotype. Besides the fact that every clock produces a distinct chronotype (depending on its internal day length), there is an important difference between those clocks that produce internal days shorter than twenty-four hours and those that produce internal days longer than twenty-four hours. When the strength of the zeitgeber decreases, the former become even earlier and the latter become even later. Or conversely, when the strength of the zeitgeber increases, the former become later and the latter earlier.

  Joseph grew up on a farm and continued to work predominantly outdoors, being exposed to a strong zeitgeber. When Frederic started to work in the factory, he remained predominantly indoors, and since his body clock produced an internal day longer than twenty-four hours (as in the case of most humans), he consequently became a later chronotype. The main consequence of industrialization is that people work in buildings and that more and more people live in densely populated settlements. The difference in light intensities within buildings and outdoors is enormous. While intensities in a well-lit room hardly ever exceed a couple of hundred Lux, intensities outdoors reach 10,000 Lux even on the rainiest of days and about 150,000 Lux on cloudless days.2 But not only do town people get less light during the day (because they are mostly indoors), they are also exposed to more light during the night because cities are more brightly lit than rural regions. Thus, the amplitude of the light–dark cycle is much smaller in towns than in the countryside.3 The stronger a zeitgeber, the more efficiently it compresses or expands the internal day.4 Consequently, different zeitgeber strengths also create different response characteristics so that the clock has to change the rules of its hide-and-seek game. The top panel in the graph on the next page may look familiar from our discussion of days on other planets. It represents a body clock that produces internal days longer than twenty-four hours.

  Since a stronger zeitgeber will increase the compression area more than the expansion area, the clock has to hide more of the former in darkness. As a consequence, we become earlier chronotypes when we are exposed to more light during the day (and to less light during the night). Inversely, if the zeitgeber strength becomes weaker, individuals become later chronotypes, as Frederic did when he started to work predominantly indoors.

  You probably have experienced how much earlier you can fall asleep after having spent a day in the country, at the beach, or in the mountains. A German proverb says, “Fresh air makes you tired.”5 Greater physical activity and fresh air can certainly contribute to exhaustion, but being utterly exhausted didn’t necessarily help Sergeant Stein to fall asleep promptly. One reason why we can fall sleep earlier after having spent a day in the “fresh air” is presumably because we were exposed to brighter light—even on a rainy day—and bright light will more efficiently compress the internal day and thus make us earlier.

  Above, a body clock that produces internal days longer than twenty-four hours. A stronger zeitgeber will increase the compression area more than the expansion area (below).

  So far, we have considered the situation of a body clock that produces internal days longer than twenty-four hours. The rule for these cloc
ks is: the brighter the light during the day, the earlier the chronotype. But why then did Hanna go to bed earlier and not later when she stopped leaving the farmhouse during her depression? She is an extreme early type whose internal days are shorter than twenty-four hours. She therefore became even earlier when her zeitgeber became weaker. Joseph, Frederic, and Sophie intuited correctly that Hanna’s sleep habits would improve if she helped more actively on the farm and thus spent more time outdoors. More zeitgeber strength would allow Hanna to stay up long after 6 P.M.

  In the preceding chapter, you read about the east–west gradient of chronotypes that almost perfectly matches the progression of the sun. However, I have not shown you all the results of this study. The systematic east–west gradient in Germany shown in that chapter was composed only of locations with up to 300,000 inhabitants. The graph on the following page shows that in cities with a population of up to half a million (squares in the graph), chronotypes are later and the slope is flatter than the migration of sun time from east to west (dashed line). The average chronotype of people who live in even bigger cities (more than half a million inhabitants—shown as triangles in the graph) tends to be even later, and the slope is even flatter, even though the systematic east–west gradient is still significant.

  This chapter helps you to understand this phenomenon. People in rural regions are exposed to a stronger zeitgeber compared with those living in big cities and are therefore earlier as well as closer to the progression of sun time from east to west. You can even see this phenomenon in regions with less than 300,000 inhabitants (the gray circles). The slope in the east (right half of the gray circles) is steeper than in the west (left half of the gray circles). This is true not only for the whole of Germany (including the states of the former GDR) but also for the southern states of Baden-Wuerttemberg and Bavaria. A much better explanation than culture for the different slopes is population density: its increase from east to west is steeper in the eastern half of the country than in the more populous west.

  The influence of light as zeitgeber for the body clock depends on population size. The larger the city, the later the chronotype. But even big cities still show a significant east–west gradient.

  One of the questions in the Munich ChronoType Questionnaire asks participants to indicate how much time they spend outside every day, without a roof above their head. It is remarkable how little time many people spend outside of buildings or some kind of vehicle. According to our survey, about half of the Central European population spends on average less than an hour outdoors on work days and less than three hours on weekends. When we correlated the time people spend outdoors with their chronotype we found a very systematic relationship that can well explain the differences in sleep–wake behavior between town and country folk.

  Based on these highly subjective self-assessments, spending two hours or more outdoors can advance an individual’s chronotype by about one hour. This means that if we were to cycle to work and back instead of taking the subway, we might become one hour less sleep-deprived every night during the workweek and would therefore have less sleep loss to compensate for on our free days. And we would also boost our learning capacity, our immune system, our mood, and our social skills.6

  People in industrialized settings are exposed to zeitgebers about 200 times weaker than those synchronizing the clocks of people who work outdoors. Our modern life in constant twilight has substantial consequences for the timing of our physiology. We can only guess the consequences on our health and well-being of this—on an evolutionary scale—relatively recent lifestyle.

  You have seen how different zeitgeber strengths affected the twins and their wives. The solution the quartet found to reinstate their happiness went much further than merely reorganizing the centers of gravity of the women’s occupations. I am sure you have guessed that the rearrangements also extended to their emotional centers of gravity. The four swapped partners and thereby ensured that each couple spent a lot of quality time together, both during the day and at night. The new arrangement was socially awkward among friends and family at first, to be sure. But in the end the new couples’ happiness was apparent to all. Hanna was able to enjoy more of the evenings together with the other three in the big farmhouse kitchen. Only Sophie became an even later chronotype after she stopped working outside (she eventually took over Hanna’s parents’ shop). But she did thoroughly enjoy her lonesome nights in the kitchen looking over the valley bathed in magical light around the full moon.

  19

  From Frankfurt to Morocco and Back

  Edgar Mass made little notes in his small notebook. He had been up before the sun to measure all aspects of dawn right up to actual sunrise.1 He stopped making measurements when exactly half of the sun’s disc had risen above the horizon. He added some last entries to the long lists and tables he kept in his little black notebook. His handwriting was tiny but immaculately clear. He packed away the small devices he always carried with him to track the sun during the day and the stars at night: a tiny monocular telescope of excellent quality; a small light meter; an altitude meter; and a pocket sextant. When everything was neatly stowed in a small pouch, he went downstairs for breakfast.

  Edgar had retired early from his job as a Measurement and Control Technology (MCT) technician but basically continued what he had done all his life: he measured. During his professional life he had developed a deep distrust of what other people measured. He only believed his own measurements. He was obsessed with measuring the world—every aspect of it. His heroes were the naturalists and explorers of the nineteenth century who had shown that the only path to discovery and understanding of the laws of nature was by exact quantification. All modern knowledge, he thought, had been achieved only by the exact measuring of everything.

  When he came down, Ingrid had prepared breakfast, but as usual she left the eggs to Edgar. For him, cooking the perfect breakfast egg was a ritual that considered height above sea level, air pressure, and other influences. “Good morning, Ingrid,” he said, entering the kitchen. “Sun came up at the right time again.”

  “Fascinating, dear,” she replied, and gave him a flash of a smile without directly looking at him.

  “You know how comforting it is to see the sun always come up at exactly the time it is supposed to?” he continued. “183 seconds earlier than yesterday.”

  “How interesting, Edgy dear,” she repeated, and flashed her broad smile again.

  “Since we have to leave soon, I think we should skip the eggs today,” Edgar said.

  Ingrid replied, “As you wish, Edgy.”

  They would be leaving the house for the airport in less than an hour to catch a plane for Casablanca, from whence they would continue their journey by bus to southern Morocco. Ever since Edgar had retired, they spent their summers in a small apartment that had a roof terrace overlooking the ocean. Edgar found this spot ideal for measuring the movements of the stars, the planets, the moon, and the earth. The accuracy of the celestial players gave him a deep feeling of security. He always chose the day of their annual departure to fall just before Europe switched the clocks to daylight saving time. Edgy detested the human hubris of messing with the sun’s time. He always felt like a proud fugitive, who left his country like those who had to leave for political reasons.

  A couple of hours later, they were crammed into their economy-class seats, trying to master the almost impossible task of juggling the multitude of objects on their miniature foldable trays: containers filled with typical airplane food; plastic glasses containing water and juice; two cups of coffee; Ingrid’s crossword puzzle; and several of Edgy’s measuring devices, one of which was an altitude meter. The captain had just turned on the intercom and was providing the passengers with crucial information about the weather, their estimated flight time, and the local time at the destination. “We are cruising at an altitude of 30,000 feet,” he said, concluding his address to the cabin. “Enjoy your meal.”

  Edgar looked at his altimeter
and frowned. He turned to Ingrid. “But that’s not correct, our altitude is 29,700 feet,” he said. “This is bad; if their devices are incorrect, we might crash into another plane.”

  “Are you sure?” Ingrid looked worried, but Edgar gave her a withering glance, as if to say: “Have I ever been wrong?” Edgar pressed the call button, and when the stewardess arrived he told her that he needed to speak to the captain on a matter of life and death.

  The first response of the stewardess was panic. But looking at Edgar she decided that he was an unlikely candidate for a terrorist, so said, “I’ll see what I can do.” A couple of minutes later the co-pilot came from the cockpit, and Edgar told him that their altimeters were obviously not working properly. He was very relieved when he was told that the captain’s declaration was an approximation and that Edgar’s measurements were, of course, right on target. He needn’t worry, the co-pilot reassured him, because 29,700 feet was exactly the height the plane should be flying at. Ingrid pretended to be deep into her crossword puzzle during the entire incident.

  The rest of the trip was uneventful, except for the little incident at the currency exchange counter, where Edgar challenged the conversion rate. They arrived at their summer home late in the evening and Edgar immediately went up to the roof terrace to take some measurements before retiring. After a short night of sleep, he was back up there again before dawn. While finally packing away his instruments into his small pouch, he heard someone come up the stairs leading to the rooftop. He was slightly confused because Ingrid normally would still be fast asleep at this time of day during their African sojourn. Still bending over his pouch with his back toward the opening where the stairs ended on the rooftop, he greeted her in his usual way. “Good morning, Ingrid, sun came up at the right time—at least here in Morocco.” Since she didn’t answer, he continued, “Same local time as back home, by the way, which means dawn was later than yesterday! It’s spring, the sun should come up earlier every day, not later—it’s all so wrong!” He turned around, anticipating Ingrid’s usual reply: “Fascinating, dear!” Instead she just stood there quietly and, to his surprise, was looking directly in his eyes while producing her usual flash of a smile.