Health effects of shift-work and jetlag
Health effects of shift-work and jetlag
Nearly 20% of the population in the industrialized nations is involved in shift-work! Surveys show that only 10% of the shift-working population have no complaints about the negative impact of their sleep schedules on their health and life. With well-designed shiftwork, those numbers could look much better. This would not, naturally, change the fact that all forms of sleep regulation are risky and potentially unhealthy. Research shows that shift-workers suffer from various gastrointestinal and cardiovascular problems. Cardiovascular changes might be mediated by inflammatory markers such as C-reactive protein. Many have problems with achieving refreshing sleep. After many days of chronic sleep restriction, a significant degree of cognitive decline accumulates. This decline leads to levels that in the end approach those found in severe acute total sleep deprivation. Substance abuse among shift-workers is also much higher than average. Seemingly minor problems such as headache, inattention, decline in libido, fatigue, irritability, etc. all add up to pretty miserable life for a vast majority of workers on a poorly designed shift schedules. The set of problems affecting shift-workers is pretty familiar to researchers studying jetlag. Separate medical terms have been coined for the two related sets of symptoms: shift work disorder (SWD) and jet lag disorder (JLD). The most dramatic finding in reference to jetlag was the loss of cells in the hippocampus in flight attendants who were employed for longer periods in jobs involving intercontinental flights (Cho 2001; Cho et al. 2000). We can surmise that the exactly same health issues (times ten) would affect polyphasic sleep adepts if they could only last on their schedule long enough.
In addition to the direct effects of sleep phase misalignment, there is also a degree of sleep deprivation in shift-work and jetlag. Sufficient sleep is important for proper glucose metabolism and prevention of obesity and type II diabetes. Sleep restriction decreases the levels of leptin and has an opposite effect on ghrelin. Those two appetite hormones, as a result, make sleep deprived individual feel hungrier than well-rested individuals and shift upwards the set point of body fat weight in the caloric balance homeostat. Restricting sleep to 5 hours per night causes some 20% change in the levels of these appetite control hormones. This change corresponds to some extra 1000 kcal in free running feeding, or over 3 kg of fat per month in energy terms. Sleep restriction can easily halve insulin sensitivity leading to type 2 diabetes. It also significantly increases the risk of hypertension, stroke, heart attack or kidney failure (Van Cauter et al. 2007). Other hormonal changes include increase in thyroid hormone levels (Allan and Czeisler 1994), prolactin, LH, and estradiol (Baumgartner et al. 1993). Finally, the root cause of many phase shift problems is a complex impact of shift-work and jetlag on the circadian changes in the level of the stress hormone cortisol. The net effect of the impact of cortisol level changes is the hypercatabolic state that effectively results in the body "eating itself up" in the long run. This way, when neglecting your body clock, you can become obese and biologically "wasted" at the same time.
In 2007, the International Agency for Research on Cancer issued a statement saying "Shiftwork that involves circadian disruption is probably carcinogenic to humans". Using the term "carcinogenic" is probably slightly misleading as the actual cause of increased cancer in shift-workers is probably related to the decline in the immune function and the body's natural ability to fight off mutating cancer cells. However, the statement is important as it seals the fate of shift-work and jetlag, which should ultimately fall into the category of long-term health risk factors that cause wide ranging and serious systemic health problems.
For more about the tiny and delicate structure of the body clock, see the section devoted to the suprachiasmatic nucleus.
Properly designed shift work
I often qualify shift-work as a health risk with the designation "poorly designed". This is because it is possible to design schedules for a group of people where the circadian disruption is minimum. Using chronotherapy it is possible to gradually phase in employees into working through the night. The chief principle of such a therapy is that phase shifts should not exceed one hour per day and should, with few exceptions, be forward shifts (i.e. shifts where the days are longer than 24 hours, not shorter). All therapies that depart that principle and involve leisure time, napping, bright light, melatonin, sleeping pills, modafinil, etc. are a pure waste of time as they keep fighting the inevitable: a misalignment between the work time and the subjective night period. This misalignment can only be remedied by a gradual properly timed phase-shift-based adjustment.
Even though many shift workers will disagree with me (mostly for psychological and convenience reasons), I insist that it should be easier and healthier to maintain a night shift for a longer period (e.g. a month) than to do regular cycling between night and day without the body clock having any chance for adjustment. Some cancer researchers also oppose long periods on night shifts due to the documented decline in melatonin that is believed to have cancer protective properties. However, those need to be weighted up against an even more serious problem of the circadian disruption.
One of the most persistent myths about sleep is that our body is programmed to get as much sleep as possible. Even some reputable researchers subscribe to this idea! They compare sleep to overeating. Some note how long Inuit sleep in winter. Others note that people allowed to sleep freely often binge heavily and clock up an indecent number of sleeping hours. As if conservation of energy was the main function of sleep. As if all animals were made as lazy as they are perpetually hungry.
Some scientists even contemplate sleep restriction analogous to calorie restriction. It is conceivable that sleep restriction might be helpful in some rare cases in sick people (e.g. "wake up to get your medicine"). However, it's analogy to calorie restriction is as weak as the reverse proposal: wake restriction. The myth was probably born from epidemiological studies that show that people who sleep 7 hours per night live longer than those who sleep 9 hours per night. However, the suggestion to restrict sleep to live longer is as smart as an effort to shrink or stretch people just because those who are very short or very tall do not live as long as an average man in the street.
We can't demonstrate any evolutionary advantage to getting more sleep than neurally necessary. The harmful myth of excessive sleeping might make you think that free-running sleep will make you sleep longer in the same way as free access to the kitchen will make you overeat. Considering the known functions of sleep, there is no specific benefit to sleeping beyond the standard 6-8 hours. Sleep is a neurophysiological consumer of benefits accumulated in waking (such as learning, exercise, etc.). Its healthy homeostatic and circadian control roughly ensures the optimum proportion of sleep to waking. People who binge on sleep in free-running conditions usually come from a period of long-lasting sleep deprivation or initiate sleep too early in reference to their circadian phase. Their total sleep time quickly drops to their natural average after a couple of days on a free schedule. A study showed that to get over 8 hours of uninterrupted sleep, the sleep should be initiated some 6 hours before the temperature nadir (shortly after the alertness acrophase)(Dijik and Lockley 2002). The same can be seen in SleepChart data submissions. For example, in the presented graph, the maximum length of sleep is obtained when sleep is initiated 3 hours ahead of the most favored bedtime (merely an hour after the evening "forbidden sleep zone"). Those observations have put paid to the idea that we have a tendency to sleep excessively.
Circadian graph that shows that "excessive sleeping" occurs when sleep is initiated too early. In the graph, sleep initiated in the 16th hour is longer than average, while the sleep-wake cycle does not add up to 24 hours (unbalanced cycle with phase advance). In contrast, sleep initiated in the preferred 19th hour is nearly an hour shorter and produces a perfectly balanced 24-hour sleep-wake cycle.
If your main concern is time, you can survive on less sleep and get more time at the cost of your mental acuity. If your main concern is the brain power, you should live by the motto: Maximum efficiency of sleep is accomplished when sleeping without artificial sleep regulation (i.e. without alarm clocks, pills, designer schedules, substances, etc.). Free-running sleep schedule will make you sleep less on average. It will make you sleep much less than on any artificial sleep schedule that forces you to catch up with the accumulated sleep debt. Irregular schedule is bound to produce deficits because you can accomplish irregular sleep only by interfering with it. To read more about excessive sleeping see: Jim Horne and Daniel Kripke.
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