In circular time structures, simple, orderly sequences are difficult to determine—the old chicken-and-egg problem. Is dawn before dusk or is dusk before dawn? Is midnight before noon or is noon before midnight? It seems easier when events lie closer to each other: dawn occurs before noon, dusk after noon. But even these apparent certainties are not necessarily correct. If the cooling-off at the end of the day influenced the weather on the next day, then the rain at lunchtime would be (partly) “caused” by the prior dusk, which would then be before and not after noon—the day is indeed a circular structure. This temporal chicken-and-egg problem is a good reason for challenging the early-bird wisdom. As long as all individuals have similar daily routines (in other words, as long as they are similar chronotypes), the earliest bird has an advantage over anyone getting up later. This was probably true for most preindustrial societies; hence the persistence of the early-bird proverbs. If the distribution of chronotypes becomes as broad as that shown in the first chapter for Central Europe today, then the temporal chicken-and-egg problem starts to apply to the hunt for resources. A small number of very early birds in the population would wake up on their own between four and five in the morning, but even more very late chronotypes would still be awake. There is no reason why these extreme late types couldn’t gather all the mushrooms before the early risers arrived in the forest. They could then go to bed and sell the mushrooms to the early birds in the afternoon. They would even have a monopoly on mushrooms because the next good crop wouldn’t have grown until the next morning. If you have difficulties with the mushroom metaphor, then think of the impact that stock exchanges have on each other. The last stock exchange of the day, Wall Street, influences the first stock exchange of the next day in Tokyo, which in turn will have an impact on all the others between Tokyo and New York.
This myth that early risers are good people and that late risers are lazy has its reasons and merits in rural societies but becomes questionable in a modern 24/7 society. The old moral is so prevalent, however, that it still dominates our beliefs, even in modern times. The postman doesn’t think for a second that the young man might have worked until the early morning hours because he is a night-shift worker or for other reasons. He labels healthy young people who sleep into the day as lazy—as long sleepers. This attitude is reflected in the frequent use of the word-pair early birds and long sleepers (as mentioned by the journalist).7 Yet this pair is nothing but apples and oranges, because the opposite of early is late and the opposite of long is short. Although duration and timing are the two major qualities of sleep, they are independent from each other. Sleep duration shows a bell-shaped distribution within a population comparable to that of sleep timing, but in this case, more short sleepers are on the left of the distribution than long sleepers on the right.
Almost a quarter of the population sleeps around eight hours (averaged over work and free days); close to 60 percent need between 7.5 and 8.5 hours of sleep (the three most populated categories in the graph). People who get by on less than five hours are very rare (but do exist), as are those who need more than ten hours every night. Due to the different sleep needs of individuals, the concept of midsleep was introduced to characterize when people sleep, which also gives us an indication of the relationship between an individual’s internal time and local (external) time. There are just as many short and long sleepers among early chronotypes as there are among late chronotypes; or turned around, there are just as many early and late chronotypes among the short sleepers as there are among the long sleepers.
Thus, the notion that people who get up late sleep longer than others is simply wrong. This judgment presumes that all people go to bed at the same time, which we know isn’t true—certainly not in the world we live in today. But what is it that makes us fall asleep? Is it merely a signal by our biological clock? Surely not, otherwise we couldn’t have an afternoon nap or a siesta. There must be more to falling asleep.
Sleep duration shows a bell-shaped distribution within a population, but there are more short sleepers (on the left) than long sleepers (on the right).
3
Counting Sheep
Sergeant Simon Stein lay down on one of the mattresses lined up in long rows on the floor of a big windowless gym and wondered why he had signed up for this project. He and thirty-four other soldiers had to perform a lot of psychological and physical tests at different times of the day over a certain period—exactly how long the project would last was still an open question. They were allowed to sleep one-third of the time but were tested during the rest. When first proposed, this sounded almost better than their usual routine, because Stein’s unit often had to work shifts that lasted much longer than two-thirds of the day and began at odd, constantly changing hours.
He had finished his first series of tasks and was now allowed to lie down and sleep. A siren gave them just enough time to get comfortable for their rest period before the lights went out in the big gym. Now that Sergeant Stein was lying in the dark on his mattress, however, he couldn’t find sleep, and so he reflected on this first testing period. He remained awake, and before he knew it, another siren summoned him and the other volunteers for the next round of tasks, shortly after the overhead lights had gone on again.
The tasks during the light period were not too difficult. Some of them were actually quite a lot of fun. There were reaction-time tests where they had to press buttons, a left or a right one, to match two little lights mounted on a vertical board in front of them. They had to estimate time spans, in the second and minute ranges, or they had to use their full muscular strength pressing a flexible barbell. They had to cross out all the p’s on a sheet filled with p’s, q’s, and d’s. They had to do simple arithmetic, memorize shopping lists, or ride a stationary bike as fast as they could. The test periods were crammed with so many tasks that Sergeant Stein usually felt as if they were over before they had started.
The alternation between light and darkness, between rest and activity, had repeated itself for several cycles now, and since Stein had hardly ever managed to fall asleep during his rest periods—no matter how many sheep he counted—he began to feel pretty tired. Finally, a couple of cycles later, he fell asleep almost before his head hit the pillow and slept like a log until the siren and the lights made him sit upright with a jerk.
Although he was quite sure that he had fulfilled all of his tasks as well as he had done at first, he somehow got the feeling that his supervisors were not as enthusiastic as they had been. Officially the volunteers didn’t receive any feedback from the supervisors, but Stein was an observant man and he was sure that the attitude of his supervisors had changed. Maybe they were also worn down, although they frequently spelled each other, unlike the test subjects.
The longer the project lasted, the more tired he became. It was astonishing that he was unable to get a wink of sleep during some cycles but was able to sleep the entire rest period during others. He lost track of how many days they had been alternating between performing tasks and more or less successfully getting some sleep between the testing cycles. But he was starting to see a pattern in his changing capacity to fall asleep. He was puzzled by the fact that, despite this insight, he was unable to sleep during some cycles—because he was more worn out than he had ever been in his life. His mates were all experiencing similar difficulties and patterns of exhaustion. Finally, the project was terminated because the volunteers were deeply exhausted.
Sergeant Stein’s story represents many different experiments that investigate how different qualities of an individual change in the course of the day. In these types of experiments people live under very controlled conditions for several days and perform countless tests that probe performance, reaction time, and muscle strength across the twenty-four-hour day. Stein’s experiences are closely related to an experiment performed by the Israeli military. The military powers of all nations are extremely interested in understanding what makes humans sleepy and, even more important, why lack of sleep affects
us so profoundly and so negatively. In combat situations, not needing to sleep or taking a drug that prevents sleep-loss from interfering with performance would provide an obvious advantage.1
Chronobiologists, those researchers who investigate biological clocks, are mainly interested in sleep because its timing is the most conspicuous expression of the body clock in humans and other species. There are many other fascinating aspects of sleep besides its timing, such as sleep structures, the function of sleep, sleep pathologies, or the relationship between sleep and the immune system, to name just a few. Many of these aspects are scrutinized by sleep researchers. Chronobiology and sleep research used to be entirely separate disciplines, but their representatives came to recognize that both groups can learn and profit from one another, and nowadays they often meet at the same conferences. An excellent example of teamwork that brought the two disciplines much closer together is the theory about how sleep is regulated. It was developed by the sleep researcher Alex Borbely and the clock researcher Serge Daan.
Both knew that sleep is regulated by at least two independent components—by being tired and by time of day. The first component was known long before people ever heard about biological clocks. The longer we are awake, the more tired we get—it’s common sense. But even before the discovery of the biological clock, people must have found themselves lying awake despite utter exhaustion (like Sergeant Stein). Borbely and Daan consolidated the two reasons why we fall asleep into one relatively simple model. This model involves two different kinds of rhythm produced by two different kinds of oscillators, one behaving like an hourglass and the other like a pendulum.2 While an hourglass has to be actively turned once its top chamber is empty, a pendulum swings by itself (forever, if it is kept in a vacuum).
In Borbely and Daan’s model, one of the chambers of the hourglass represents sleep pressure. During wakefulness, this chamber is at the bottom and is continuously filled with “sand.” When the level of sand reaches a critical threshold, we fall asleep. The hourglass is turned around, and the chamber empties until it reaches another (lower) threshold that wakes us up; the hourglass is turned around again, and the cycle restarts. These alternating processes create a sawtooth pattern.
So far so good, but this model doesn’t explain why we can have a long sleep at some times of day, even if we are not exhausted, whereas at other times, we can only get a short nap or are even incapable of falling asleep, despite being totally exhausted. The model also cannot explain why we always sleep more or less at the same time of the day (or rather night). If we stayed up beyond the upper threshold (depriving ourselves of sleep) just once, we would—according to this model—go to bed a little later in the evening for the rest of our lives.
The sawtooth pattern of the sleep–wake cycle.
Staying up just a little too late—hypothetically.
This doesn’t fit with our experience. Despite wiggles and larger perturbations in our bedtimes, most of us tend to fall asleep within a relatively stable time frame, with the majority of the population sleeping while our part of the globe is turned away from the sun. Daan and Borbely solved this problem by postulating that the thresholds that switch our body into sleep and back to wake oscillate with a daily rhythm.3
This model now explains why sleep deprivation does not permanently shift our sleep timing: once we get recovery sleep, the rhythmic shapes of the thresholds will always move our sleep back to its usual times. Thus this model is much closer to what we experience in real life.
Now that you have an idea of how scientists think about sleep regulation, we can turn back to Sergeant Stein, who was allowed to sleep one-third of every cycle. Why did he get so tired? Don’t we all, on average, spend about one-third of our sleep–wake cycles asleep? That alone cannot be the reason he and his fellow soldiers became so exhausted that the scientists decided to terminate the experiment. Were the test batteries during the wake periods too exhausting? It’s unlikely; after all, the participants were all highly trained, fit soldiers. The reason why they became so exhausted was that they couldn’t get enough sleep over the course of many cycles. Their experimental “days” (or cycles, as I call them) were only thirty minutes long. They worked on tests for twenty minutes, then a siren signaled them to go as fast as they could to their mattresses, then the lights went out and they were allowed to sleep for ten minutes. If sleep were regulated purely by the average time a person was awake, the soldiers should have had nothing to complain about because, on average, the proportion of sleep to wake matched the usual proportion outside the study.
But sleep regulation is more complex: they didn’t sleep during the first hours of the experiment because their sleep pressure hadn’t reached the upper threshold. Further into the experiment their sleep pressure reached the point that triggered sleep, but instead of sleeping for eight hours, they were woken up after ten minutes for another twenty-minute round of tests. What the model of Daan and Borbely doesn’t show is that there are times during our internal twenty-four-hour day when we cannot readily fall asleep, despite being utterly exhausted. These times usually occur during our daytime, and those were the times when Sergeant Stein couldn’t find sleep during the allotted ten minutes. He might have fallen asleep later, but that was prevented by the experimental plan. In the end, the soldiers were so sleep-deprived that the experiment was terminated.
The thresholds that switch our body into sleep and back to wake oscillate with a daily rhythm.
This rhythmic threshold helps our sleep to stay synchronized at its normal phase, even when we stay up too late.
Interestingly, there is one time during our daily cycle when we are least likely to fall asleep, and that is shortly before our body clock opens the temporal window that allows us to sleep best. It almost seems that this time is a forbidden zone for sleeping.4 This zone was first discovered by the experiment described in our case story.
Sleep deprivation has often been used as a form of torture, to make prisoners confess their supposed crimes. In most cases that torture works really well. The only drawback is that prisoners get so exhausted that they practically lose their minds and start to confess to things they never did. I once received a phone call from a police interrogator who wanted expert advice about how and when he could use sleep and time of day to weaken the defense mechanisms of a suspect. I told him (in somewhat politer words) to go to hell or, even better, to call Amnesty International.
This chapter has covered the different factors regulating sleep. We don’t fall asleep just because we are exhausted; there is another factor that controls our capacity to fall asleep and to benefit the most from our sleep, and that factor is the biological clock. But what is this biological clock? How does it work, and how important is it for us in our daily routines? And who was the first scientist to discover this extremely important part of our biology?
4
A Curious Astronomer
On a lovely evening toward the end of the summer of 1729, the French astronomer Jean Jacques d’Ortous de Mairan sat at his desk working on a manuscript. He paused to think about a difficult sentence. As some of us do when we concentrate and turn our thoughts inward, he gazed out of the window into the cloudless evening sky. Although it was still bright enough to work without a lamp, the quality of the celestial light clearly announced the approaching night. He didn’t manage to capture his thoughts in words that his readers would unmistakably understand. His inward gaze therefore slowly came around to focus on the world in front of his eyes, which fell upon a plant with delicately feathered leaves growing in a pot on his windowsill. The mimosa was one of his favorites. Now that his attention had returned to reality, de Mairan saw that the mimosa had already “gone to bed” by furling its leaflets, which had been stretched out throughout the day. He envied the plant for its capacity to fall asleep with unbreakable regularity and precision. He wondered whether one could compare the up and down movements of plant leaves with wake and sleep in humans—which unfortunately did not show the same reg
ularity for him. Particularly when he was writing, he worked into the small hours, until he was too tired to hold a quill. Even then he would lie awake for a long time with his thoughts whirling around the manuscript until he finally fell asleep, waking only well after the sun had risen. This particular evening, while trying to find sleep, his mind got hooked on the question of whether sleep and wakefulness had any similarity with the daily up and down of his mimosa’s leaves. He was sure that most animals slept, so why not plants? Yet plants didn’t run around, and they didn’t exhaust themselves like animals did, so why should they recuperate in sleep? Being sessile, they couldn’t escape the sunlight like animals. They were completely dependent on light and darkness. Maybe it was strenuous for the plants to keep their leaves in a horizontal position. Did they expand them during the day to catch the sunlight or to shade their lower parts? In either case, it was unnecessary to keep them up once the sun had set. The leaf movements evidently had something to do with light and darkness. But why were the leaves then already folded down before the sun had set? Obviously the direction of the sun changed over the course of the day (for example, the sunlight didn’t come directly from above in the early evening hours), so it was advantageous to adapt the leaves’ position, whether to catch light or to create shade. He had often observed how plants turned their leaves toward the sun, tracing its path across the sky. The more he thought about the phenomenon of leaf movement, the clearer it became to him that it all depended on sunlight and darkness.
Suddenly he was wide awake, even though it was 3 A.M., and he sat up quickly. He had an idea about how to determine if the leaf movements were merely passive reactions to the sun. He got out of bed and ran downstairs to his study. Frantically, he opened the door of his desk cupboard and pulled out all the drawers to make space for the mimosa, placed the pot inside, and closed the door. He was so excited about his simple experiment that he feared he would not sleep at all that night, but once back in bed he fell asleep immediately. It was well into the next day when he woke. It took him quite some time to remember that he had started an experiment the night before. He jumped out of bed, went to his study, and closed the curtains to make the room as dark as possible. Then he cautiously opened the desk door just wide enough to get a glimpse inside. To his great surprise, the mimosa was “awake” and had fully extended its leaflets to their usual daytime position, even though they had been in complete darkness!
Internal Time: Chronotypes, Social Jet Lag, and Why You’re So Tired Page 3