There is no doubt that shift work is detrimental to health, but the mechanisms linking shift work and pathology are far from well understood. The issue is complex. Shift work means eating when the body is not optimally adjusted to digest food and not eating when the stomach is prepared to do so. It means sleeping outside of the sleep window provided by the body clock and working during our internal night. It means being exposed to light when the eyes “expect” darkness. It means not being around when a partner and children end their day or when they get up in the morning. It means trying to sleep while the children want to be active, when the streets are full of noise, and when it’s bright daylight outside. It means constant disruption of one’s social life and being exhausted even on free days. (On free days, shift workers try to fall back into synchrony with family and friends.) It means stress for body and brain, leading to coping and compensation attempts—for example, by consuming large amounts of caffeine or by smoking many cigarettes, as we know people do when they suffer from social jet lag.
Initially, shift work was mostly confined to professions responsible for our health and safety (hospitals, police, fire brigade, and so on). Then it spread to the production industry, where expensive machinery needed to be utilized around the clock. More recently, shift work has reached service providers like the call center described here. One would think that the call center of a bank would keep relatively normal working hours, maybe with extensions outside of the traditional nine-to-five arrangement. After all, few customers want to make transactions at 4 A.M. The reason for shift work in the service sector is outsourcing. Millions of people in India, the Philippines, and other Asian countries work in the Business Process Outsourcing (BPO) industry. In the metropolitan region of Manila alone, more than 240,000 people work in call centers and other BPO facilities. When a British bank outsources its telephone services to another country, the service still has to be provided during British working hours. Nine to five in Britain means 4 P.M. to midnight in Manila; and nine to five in Boston is exactly twelve hours later (or earlier) there. The employees of an outsourced call center have to pretend to live in the same time zone as their customers, and so they have to speak the way their customers do, and they have to know about the local weather at their client’s location.
Shift work means living at odds with a normal social life as well as against the body clock. It is therefore not surprising that it harms health. Although shift work may be the immediate cause of certain illnesses (for example, ulcers, because of eating at the wrong times), it is unlikely that it is the direct cause for all the pathologies that epidemiological studies have shown to be associated with shift work. A more likely scenario is that the constant physiological stress weakens the system and thereby also weakens its ability to keep disease at bay.
The light-at-night (LAN) hypothesis proposes that shift work can directly cause tumor development. This hypothesis follows a chain of six arguments, each of which are correct as individual statements: (1) melatonin is a hormone produced under the control of the circadian clock at night; (2) melatonin synthesis can be suppressed by light; (3) melatonin belongs to the chemical substance class of indolamines;1 (4) indolamines can act as scavengers of oxygen radicals; (5) oxygen radicals can damage DNA; (6) DNA damage can cause cancer.2 The idea behind the LAN hypothesis is based on the observation that the production of melatonin is inhibited by light, and since shift workers are exposed to light at times when their body normally produces melatonin, less of this indolamine will be circulating in their blood compared with people who sleep in darkness at the right internal time. The presumed reduction of melatonin in shift workers is therefore thought to cause cancer via increased DNA damage by oxygen radicals.3
Advocates of the LAN hypothesis argue that the potential of melatonin to reduce cancerous growth has been shown in animal experiments. However, these experiments rely either on cancer cells already being present, or on such cells being purposely injected into the animals. Thus, if someone had already developed a tumor, shift work could certainly promote further cancerous growth. But that doesn’t mean that shift work actually caused the cancer in the first place. Thus, we are back to the more likely scenario; the constant physiological stress of shift workers weakens the system’s ability to keep potential pathological developments at bay.
The LAN advocates argue that epidemiological studies have shown cancer rates to correlate with the degree of light pollution. But light pollution is also a clear indicator for an industrialized lifestyle, which includes many factors that may individually or in combination contribute to increased cancer rates. Thus LAN is more likely to be a cancer predictor than a direct cause for cancer. The media frequently pick up on the proposed connection between LAN and cancer. Some media reports have even insinuated that any light pollution—from streetlamps to nightlights—could lead to the development of cancer. But even if reduced melatonin levels promoted tumor growth, it is unlikely that melatonin is suppressed in people who are active (and exposed to bright light) during the day and sleep in a relatively dark room during the appropriate internal times. For light to be effective in melatonin suppression, one would have to assume that people sleep with their eyes open in a relatively well-lit room.
Under normal circumstances, closed eyelids reduce the amount of light reaching the retina by more than 80 percent. Our reddish eyelids filter away even more (97 percent) of those wavelengths that are most efficient in suppressing melatonin. A lamp producing, for example, 100 Lux shining directly on closed eyelids would thus expose the retina to no more than three Lux. This is less than a third of the intensity that researchers use in controlled laboratory studies investigating the body clock when they want to be sure not to affect melatonin levels or other physiological parameters. But even the three Lux are an overestimation because our eyeballs roll upward during deep sleep, reducing retinal exposure even further. It is therefore extremely unlikely that melatonin is suppressed during our sleep by nightlights, televisions, computers, emergency lights, moonlight, streetlights, or any other urban light pollution. Even if we slept in a bedroom during the day with closed curtains (for example, following a night shift), melatonin should not be affected. Under these circumstances, it is unlikely that light pollution can be directly linked to health via the LAN hypothesis. Instead of being afraid of light at night, we should pay more attention to the fact that we, as industrialized beings, expose ourselves to far too little light during the day. It is more likely that the weak zeitgebers in urban settings—due to relatively dim daylight exposure in combination with more light at night—have far greater effects on our mood and on the adequate synchronization of our body clock.
One major factor in the context of the LAN hypothesis or in shift-work research in general is almost never considered: internal time. Practically all shift-work studies analyze their data based on external time. For example, night shifts in these studies start at 10 P.M., morning shifts at 6 A.M., and evening shifts at 2 P.M. Yet, the reference for all daily aspects of our physiology is individual internal time. Let’s pretend that a study wants to find out whether earlobe cancer develops more readily in shift workers than in day workers.4 The study collects epidemiological data, analyzes them, and finds no difference in the development of this pathology between shift and day workers. If, however, the scientists had assessed their subjects’ chronotype, they might have found a different result. Early chronotypes develop earlobe cancer with a higher probability if they work at night while late types have an even lower risk in contracting this pathology if they work at night. This fictional example indicates that, without the consideration of internal time, we will not fully understand the relationship between shift workers and pathologies.
In view of the lateness of most people in our modern societies, one could argue that a majority of the workforce is scheduled in a permanent early shift when they work from nine to five. The diversity of chronotypes—with extreme larks and extreme owls being up to twelve hours out of phase—predicts tha
t some chronotypes may even benefit from working at night and others would be more in synchrony with their body clock if their working days started at four in the morning. People working in rotational shifts are generally younger than the average worker. One could argue that the reason for this bias lies in the higher capacity of younger people to cope with the physiological stress of rotational shift work. Yet people become earlier and earlier in their chronotype with age and thereby may lose their endurance and their capacity to stay up late.
All the protagonists in this chapter’s story are just beyond the developmental peak of lateness and therefore can work at night more easily than older people. They can also sleep longer and better during the day after getting home. Note that this is not due to the fact that their body clocks adjust to the constant night shifts like the oilrig workers you read about. When Maria and Marco stepped out of their workplace after finishing their night shift, “they were dazzled by the bright day outside.” The light detection system of the body clock integrates light over time.5 Even if the workplace were well lit, the eyes would probably not be exposed to more than 100 Lux. In a twelve-hour shift, they would therefore integrate 1,200 Lux-hours. Let’s assume that the outside light intensity on their way to the bar was 120,000 Lux and that it took them twenty minutes to walk the short mile. In that case, they would have been exposed to 40,000 Lux-hours. But even in the rainy season, with only black clouds in the sky, they would still have accumulated 3,000 Lux-hours on their way to the bar. As a result, their body clocks don’t fully adjust to the nocturnal work hours. The reason they can cope better than older people is that their body clocks are extremely late, even if they remain synchronized to the normal daylight.
Besides the medical difficulties that are associated with shift work, there are many social issues: fathers and mothers who are out of synch with their children and their partners, as well as with their friends and relatives. As this chapter’s story described, shift workers often form subpopulations, in this case made up of youngsters who work at the same place and during the same odd hours. These subpopulations rarely mix with the rest of society. This form of social isolation has even led to the observation that the number of HIV infections in India and other Asian countries is higher among employees of call centers than in the general, age-matched population.
21
Partnership Timing
Louise was dead tired but just couldn’t find sleep. Normally she didn’t mind that Bruno was still reading beside her. But tonight he seemed to flip his pages especially loudly, and his bedside lamp appeared brighter than usual. After a lot of tossing and turning, of rearranging her duvet, of sighing deeply and frequently, she opened her eyes and turned to her husband. “Bruno,” she said in a suffering voice, “would you mind not reading tonight? I just can’t fall asleep.”
Now Bruno sighed deeply. “All right, I’ll sleep in the guest room,” he replied. He took his book and his cushion and swung the duvet over his shoulders, thereby knocking over his bedside lamp. It made a lot of noise but luckily didn’t break. He ignored Louise’s angry glower. “Serves her right for shoving me out of my own bed,” he thought. “She’s the one who can’t sleep, so why didn’t it cross her mind to retreat to the guest bed herself?”
Louise and Bruno had been married for twenty-eight years, and their children had all started their own lives outside of the parental nest. At last they had enough room for “self-actualization,” as it is called. One of the children’s bedrooms was turned into a guest room, the other into Louise’s parlor, and the third into Bruno’s den—at least that’s what the children ironically called the converted rooms. Since neither parlor nor den contained a bed, Bruno often evacuated to the guest room. Despite his grumpiness about being shoved out of his own bed, he quite liked to sleep in the guest room. It meant that he could read as long as he liked and could even listen to the radio or watch television whenever he felt like it. But even more important, he could sleep late in peace. Louise usually got up at the crack of dawn (at least in his slightly exaggerated opinion) and then opened the curtains to “check the weather.” That was the most annoying part of her morning ritual. It was she who insisted that the curtains were drawn in the first place, to prevent her waking up even earlier. He quite liked to sleep with the night’s air and the morning light coming into the room—he could sleep well into the day after sunrise. It was the sound of the opening curtains that woke him up. If it hadn’t been for the subconscious feeling of loneliness during the night, he would have taken his self-actualization a step further and turned the guest room into his own bedroom.
When he came down the next morning, he found a note from Louise on the kitchen table, announcing that she had gone off to have coffee with her best friend, Doris, as she often did on Mondays. He made himself a proper breakfast consisting of eggs and bacon on toast, which he only dared to cook when Louise was away. She had turned vegetarian after their children had left the house, while he still considered himself a true carnivore. After he had finished the last morsels, he sat on the terrace finishing his coffee and thinking about how Louise and he had become so different over the years. They had always been different, of course—it was what attracted them in the first place; but now after almost three decades of marriage it seemed that their initial differences were intensified. It had been a shallow slope from being attractively different, to occupying different and quite useful niches in their partnership, to being so different in many ways that it sometimes became a burden. It seemed to him, though, that they had already lived through the peak of discrepancies.
Before he had retired and their youngest child still (occasionally) lived at home, he sometimes thought that he and Louise lived in parallel universes. They appeared not to share a single common interest—not a book, not a radio or television program. Even their interpretation of newspaper articles went in entirely different directions. But now that he was home most of the time, it seemed that they had started to share more aspects of their lives again. They spent more time with each other and, above all, had more time to talk. In these conversations they rediscovered their common ground, even though many of their widening differences still persisted. When they sat in the car together, he had the impression that she drove more and more slowly every week, while he still liked to be an “agile member of traffic,” as he phrased it. For years her driving style had driven him bonkers, but it had become ever less important for him. In most cases, he was now even content to let her do the driving.
While he was enjoying his third cup of coffee and the newspaper on the terrace, he heard their car turn into the driveway. A couple of minutes later Louise came into the house and dumped several heavy-looking packages on the empty chairs around the garden table. “Looks like you’ve done some serious shopping with Doris,” he greeted her in a friendly tone.
“Sure did,” she replied cheerfully. “I had a bad conscience for kicking you out of bed last night and told Doris about our difficulties.” On the one hand Bruno liked the fact that Louise sort of apologized for her behavior. But on the other, he always hated the fact that Louise spoke about their private affairs with other people. But before either of these feelings could dominate his mood, Louise continued happily, “Doris laughed when I told her, because she and Sean have had exactly the same problems—but they found an ingenious solution many years ago.” She opened one of the bags and pulled out a large piece of fabric with the same pattern as their bedroom curtains. “They’ve installed a curtain that can be pulled between the two halves of their bed.” Louise held the fabric up, looking exceedingly pleased with herself.
When I give public talks about the body clock, I often start the lecture with a poll of the audience: “If the following situation seems familiar to you, please raise your hand.” Then I recount an abbreviated version of the beginning of this chapter’s story, and practically every hand in the lecture hall goes up. When I read this chapter’s story to a friend, she interrupted me after the first couple of sentences an
d said, “Have you been spying on us through our bedroom window?” Another friend just laughed: “Exactly like in my marriage, except our roles are reversed.” Divergent temporal habits seem to stand as a proxy for the many other differences that develop between partners in a long-term relationship. Triggered by an overwhelming impression that people always seem to judge their partner to be a substantially different chronotype, I wanted to collect facts, and so I turned to our database.
In the first version of the Munich ChronoType Questionnaire, we had additionally asked the subjects to assess their own chronotype and that of their partner based on seven categories.1 So for investigating partnership timing, we had the following dataset from approximately 50,000 people: age, sex, actual chronotype (calculated according to midsleep on free days), the self-assessed chronotype, and that assessed for the partner.
As you know, the chronotype of teenagers becomes gradually later until it reaches a maximum in lateness at around nineteen in women and twenty-one in men. Across most ages, men are on average later chronotypes than women. These results are shown again in the next graph. It is almost identical to the one you have seen before, except for an additional symbol: two circles connected with a horizontal arrow. They are part of a joke that I make when presenting this graph (to wake up the audience in the dimly lit lecture hall). I claim that we have not only discovered a biological marker for the “end of adolescence,” but that we have also found the reason why men marry younger women. After a little pause, I continue: “because then they can have breakfast together” and only then add the extra symbol to the graph. Since we can quite accurately calculate the chronotype of the subject (based on midsleep on free days), we can query the database about how the different chronotypes, on average, assess themselves and their partners (using the seven categories described above).2 The results for self-assessment are straightforward: the later their midsleep on free days (MSF), the later both women and men assess their chronotype, showing no differences between the sexes, as the top panel of the figure on the next page shows.
Internal Time: Chronotypes, Social Jet Lag, and Why You’re So Tired Page 18