About two-thirds of travellers report having jetlag. Symptoms of jet-lag include daytime tiredness, difficulty initiating sleep at night (after eastward flight) or early awakening (after westward flight), disturbed night-time sleep, impaired daytime alertness and performance, gastrointestinal problems, loss of appetite, and inappropriate timing of defecation and urination.
Such symptoms can seriously impair a person’s performance and ability to function, in part because of the reduction in sleep quality and quantity, and because performance and alertness rhythms will take several days to resynchronise. In the long-term (eg, after 4 years), chronic disruption of circadian rhythms from regular transmeridian travel might result in cognitive deficits (decreased short-term memory, slower reaction time) and changed physiological parameters (such as cortisol concentrations).
Because of their rapidly changing and conflicting light-dark exposure and activity-rest behaviour, shiftworkers can have symptoms similar to those of jetlag. Although travellers normally adapt to the new time zone, shift-workers usually live out of phase with local time cues.
Shift-work schedules are generally classified in terms of the speed (rapid or slow) and direction (forward or backward) of rotation. The issue of which schedules are preferable from the perspective of sleep and biological rhythm research is contentious.(1) On the one hand, in rapidly rotating schedules, which incidentally are rarely used in North America, the biological clock maintains a normal phase and workers are thus able to continue their conventional activities during off-duty days without symptoms of internal desynchrony. However, the problem with such schedules is that shifts can, and often do, coincide with the time of day when the biological drive for sleepiness is high and performance is low. By contrast, a slow rotation schedule is conducive to circadian adaptation. During days off duty, workers typically revert to the conventional day-active pattern. In Antarctica and in one North Sea oil rig shift schedule (figure 3) complete adaptation is found, but such situations are rare.(29,30) In the offshore situation, many more complications are seen in sleep and performance in the rollover shift than with 2 weeks of night shift.(31) The theoretical notion of directional asymmetry in circadian adaptation to rotating shift schedules is based on the same principles as for time zone travel; forward (clockwise) shift rotation would result in more rapid adaptation than backward rotation. To date, however, field studies have failed to conclusively show that backward rotation is more detrimental than forward.(32)
In addition to disruption of sleep, abrupt changes in time cues might have negative effects on other physiological systems. Compared with the effects of sleep, few studies have examined the effects of shiftwork on cardiovascular, digestive, immune, and reproductive systems, all of which are rhythmic in nature.(26) Epidemiological studies are problematic; we know that people who are intolerant to shiftwork tend to select themselves out of such occupations. A review of studies(34) that investigated shift work and risk of cardiovascular disease claimed that on balance, shift-workers have a 40% increase in risk. Investigators have shown that meals taken during biological night (or during an unadapted night shift) lead to higher plasma triacylglycerol concentrations (an independent risk factor for heart disease) than identical meals taken during the day, which might in part explain the increased occurrence of cardiovascular disease among shiftworkers.(35,36) Glucose tolerance is also known to deteriorate in the evening,(37) and there is evidence that increased peripheral insulin resistance might contribute to this effect.(6) Resistance to insulin is a putative risk factor for cardiovascular disease and type 2 diabetes mellitus, and again, this could explain the raised incidence of disease among shiftworkers.
Strategies have been developed to enhance circadian adaptation to shift-work schedules and time zone changes. Factors that promote sleep hygiene are advised, such as adequate sleep, sleep in a quiet and dark environment, control of the use of caffeine and alcohol, and timing sleep (with or without the use of hypnotic agents) to the desired sleep time relative to the new time zone or shift schedule. As described earlier, exposure to light can phase shift circadian rhythms. Therefore, scheduled bright light exposure and avoidance of light (possibly by use of dark goggles) might be useful in accelerating adaptation.(38) Most field studies and laboratory-simulated phase-shift studies report that correctly timed administration of the hormone melatonin is also able to moderately shorten the time taken for circadian adaptation.(26) However, there is little evidence for optimum dose or formulation, and there is no information on long-term safety. Further research is needed to examine how combined administration of bright light and melatonin could be used to develop effective, reliable, and practical treatment strategies.
It is not always desirable to adapt the circadian system to new shift schedules, for example in rapidly rotating shifts, because sleep and activity on rest days will be compromised. Similarly, when travel to a new time zone is for a short time (eg, 1 or 2 days), circadian re-adaptation might not be worthwhile. In such cases, short-term strategies can be used to maintain alertness and performance, especially during early morning hours, and to improve sleep, without shifting the biological clock.