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Evidence from the articles on the effect of the direction in shift rotation

4. SHIFT-WORK AND THE HUMAN CLOCK –

4.7. Discussion

4.7.1. The Direction of Shift-Work Rotation

4.7.1.2. Evidence from the articles on the effect of the direction in shift rotation

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The discussion on the effect of the direction in rotation is, as can be seen from the examples above, is marked by controversies and inconsistencies. These data further indicate that the underlying mechanisms are more tangled as some scientist might have initially thought.

Therefore, the articles selected for the shift-work literature survey (n=52) have been examined in respect to the direction of the shift-schedule rotation, and as already mentioned above, there is evidence of a difference in effect between clockwise and counterclockwise rotation.

4.7.1.2.

Evidence from the articles on the effect of the direction in shift

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Figure 27 Distribution of the 4 major health categories sleep (blue bars), cardiovascular (red bars), metabolic (green bars) and cancer problems (purple bars). The results are presented by the shift rotations, namely clockwise (forward), counterclockwise (backward) and fixed night work. The results from studies that did not name the direction of shift rotation are grouped under ‘not defined’.

Figure 28 Distribution of the 2 most consistent health outcomes sleep and cardiovascular problems. The results are presented by the different shift rotations, namely clockwise (forward), counterclockwise (backward) and fixed night work. The results from studies that did not name the direction of shift rotation are grouped under ´not defined´.

The separate analysis of these studies on the direction in shift rotation revealed an age difference for the average ages of the studied workers. Table 5 shows the average ages of the workers, separated by health category and direction of the shift-work schedule. For reasons of completeness, the average ages of the studies from all four categories (sleep, cardiovascular, cancer and metabolic problems, respectively) and also the ages for night workers have been added to the table.

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Rotation / health topic clockwise counterclockwise fixed night work

Sleep problems 38 36 35

Cardiovascular problems 36 41 42

Cancer diseases NA NA 47

Metabolic problems 39 49 38

Table 5 Presented are the average ages of the workers by the 4 main health categories. Most evident is the age difference of the workers studied on cardiovascular and sleep problems (36 and 41 years, respectively) for the both shift-work directions clockwise and counterclockwise (NA=no ages, due to no available studies for this group, because cancer has not been studied in other shift schedules than night work).

From Table 5 it can be seen that certain age differences appear in the groups. The importance of these differences will in the following section be discussed.

Why might this age difference be important?

As already said, clockwise rotation, compared to counterclockwise rotation, is often preferred (by both employees and employers, for mostly subjective reasons) as the latter leads earlier and to more sleep problems, social disarrangements and it thereby makes it difficult for some workers to arrange both work and family life. In addition, counterclockwise rotating systems are worked in many cases by newly employed and therefore assumingly most likely by younger workers. Table 5 shows that sleep problems appear about two years earlier in counterclockwise than clockwise rotation, which is in line with the finding mentioned above.

Pronounced night work also shows to lead earlier to sleep problems. Interestingly, the opposite holds true for the finding of cardiovascular problems, and slightly also for metabolic problems, as in these studies the workers in counterclockwise rotation are the oldest (on average). The point that I want to make here is, if counterclockwise rotation forces workers to leave shift-work early in their career, the clockwise workers from these studies cannot be former counterclockwise workers (hypothetically spoken), as these are already older (36 vs.

41 years, respectively). Based on these simple observations, the direction of shift rotation can be evidenced to play a role in the aetiology of both short- and long-term health outcomes in workers, found here for sleep and cardiovascular problems, respectively. As these shift-workers can be assumed not to differ in respect in their overall exposure to risk factors and as also the course in development of cardiovascular problems can be assumed to be equal in both groups, the only difference found here is the direction in shift rotation.

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The hypothesis for the next part of this thesis therefore is that workers employed in counterclockwise rotation are affected in older ages (later) by cardiovascular problems than their colleagues from clockwise rotation. Further, it can be hypothesised that the benefit of less sleep and social disarrangements from clockwise rotation (as reported in many subjective evaluations) might be replaced by a higher risk of developing severe heart problems on the long run. This would mean that there is a true difference in effect to the physiology by the direction of shift-work rotation that needs further investigation. Sleep problems and cardiovascular problems therefore appear as the two sides of the (shift-work) medal. Before I will come to discuss how to measure such a difference, I will first argue on the possible basis of this difference, to further illustrate the hypothesis.

What might be the basis of this difference in findings for the different shift rotations?

Cardiovascular problems develop slowly with time, mostly over years. CHD has been shown to be significantly elevated after shift-work seniority of at least 10 years, with increasing risk prevalence with increasing years on shift-work (chapter 4.5.3.3). Further, it can be assumed that (mostly subjective) first signs and indicators on heart problems are almost always not perceived by an affected person. This means, that the factors leading to disease are very likely prevalent at least for some years, but the indicators and risk factors are not always clear to the people. Studies have shown an impact of shift-work on known cardiovascular risk factors as smoking, adverse diet, altered heart rate and blood pressure, whereas this impact is found not to be shift-work rotation dependent. Therefore, the aetiology of heart disease is equal to the shift-workers, irrespective of the shift-system they are employed in. This strengthens the finding of the difference in shift rotation to have a distinct impact.

Further, in contrast to heart problems, sleep and digestive disturbances are very well noticed by a subject pretty soon (with symptoms like altered bowel habits, constipation, diarrhoea, flatulence and heartburn). This gives an affected worker directly the opportunity to establish countermeasures against the adverse impact (e.g. leaving shift-work in worst case).

In the following section it will therefore be tried to illustrate how such an individual outcome might emerge. The concept of stress will be adapted again as it has been introduced to this discussion; chapter 4.7.

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