Shiftwork - Fundamentals and Assessment Tools for Occupational Ergonomics - page 644

of the circadian system under these conditions is that it is like a symphony orchestra, with a conductor on
the rostrum making sure that the various instruments are brought in at the right time. For the night
worker, it is as if a second conductor appears on the rostrum, beating at a different time. The rate at
which the different instruments switch to the new conductor varies, and until they all do, there is a
cacophony, with all harmony lost. In circadian terms, we speak of this cacophony as “desynchronosis”
or “internal dissociation” because the component circadian rhythms no longer have appropriate phase
relationships to each other. In addition to poor sleep, the symptoms of desynchronosis include
malaise, gastrointestinal dysfunction, and performance decrements.
Individual differences in circadian system characteristics may also have a role in determining shiftwork
coping ability. Individuals who are “night owls,” or “late phasers,” in their circadian system often find
shiftwork considerably easier to cope with than do “morning larks,” or “early phasers.” 27 Phase differ-
ences may also explain why late-middle-aged people often find shiftwork difficult. A typical case is
that of a 50-yr-old patient who has hitherto been fairly happy with shiftwork but now finds it increasingly
difficult to cope with. In some ways, this is paradoxical, given that he has had many decades of learning
shiftwork coping strategies and that he probably has a quieter house now that his children have grown up
and he can afford better housing. The reason for the problem may be that he has become more of a
“morning lark” in circadian phase orientation. Carrier and colleagues 28 have shown that many of the
sleep decrements seen in the progression through the middle years of life (even in day workers) can
be attributed to age-related changes in morningness-eveningness which can occur through a person's
forties and fifties. Also, Campbell 29 has shown that circadian manipulations designed to improve
night work tolerance may work much better for young adults than for those in middle age.
Before the discussion of circadian factors is concluded, the question must be addressed whether
circadian realignment is actually desirable, given all the caveats regarding the weekend regression to a
diurnal orientation mentioned before. 12 In Europe, many companies use “rapidly rotating” systems in
which only one or two shifts are worked at a time, before a different one is worked. 30 Thus, for example,
on the “continental” rotation, employees work two morning shifts, two evening shifts, and two night
shifts, followed by two days off. Most European experts favor such systems because they allow the circa-
dian system to retain its diurnal orientation, thus eliminating problems of desynchronosis. Because only
one or two night shifts are worked before time off is given, sleep loss and fatigue are minimized. The
drawbacks of rapid rotation are the circadian-related fatigue experienced during the night shifts,
which, for some tasks, may render the approach undesirable, and the workers' difficulties in predicting
when they will be at work. However, there are undoubtedly many situations in which rapid rotation is
worthy of consideration.
32.3 Sleep Factors
Sleep is the major preoccupation of most shiftworkers. In both Europe 31 and the United States 32 , surveys
have indicated that night workers get about 10 h less sleep per week than their day-working counterparts.
Thus, individuals who happen to need 9 h of sleep per 24 h in order to feel well rested very often find
shiftwork extremely difficult to cope with. In his survey of field and laboratory shiftwork sleep
studies, Akerstedt 33,34 concluded that the shortening in a night worker's day sleep comes primarily
from a reduction in stage 2 and rapid eye movement (REM) sleep, with slow-wave sleep relatively unaf-
fected. Not surprisingly, given the prolonged levels of partial sleep deprivation involved, sleep latency can
be somewhat reduced in night workers, and some studies have found shorter REM latencies to occur.
Essentially, the problem is usually one of sleep maintenance insomnia, rather than sleep onset insomnia.
Although there are many social and domestic negatives to evening shiftwork, from a sleep point of view
such shifts are much preferable to night shifts, and even preferable to daylight shifts, particularly when
the daylight shifts have early start times.
A shiftworker's sleep loss is sometimes partially recouped on days off and by the taking of naps
but does represent a chronic state of partial sleep deprivation which undoubtedly affects the mood