Shift work can mean many things. For example, the number of possible shift sessions, the frequency of the successive shifts, as well as the single shift length, can vary. There are temporary and permanent shifts. Common in particular are two-shift systems, covering sixteen hours of the day, and triple-shift systems which enable work around the clock. Shift systems that include night work break the circadian rhythm of humans. The disruption of the circadian system is the possible base cause of the process of carcinogenesis. When the carcinogenic potential of shift work is being mentioned, shift systems are meant in which night work is also done.
Shift work is considered “probably carcinogenic to humans”
The International Agency for Research on Cancer (IARC) in 2007 made a first classification of shift work as a Group 2A carcinogen (probably carcinogenic to humans). This was based on impressive results from many animal studies and from an epidemiological study landscape which indicate risks. This classification remains where it is after the publication of the detailed Monograph: shift work at the end of 2010. Three recent studies from France, Canada and Sweden support this hypothesis further. Increased risks are shown in one of these studies for lung, bladder, pancreas, prostate, colon, rectal cancer and non-Hodgkin’s lymphoma in men. This is therefore controversial because most studies in recent years mainly indicated increased breast and prostate cancer risk. The possible ways in which the cancer is promoted are primarily by the suppression of the nocturnal secretion of melatonin, direct disruption of the so-called “clock genes” that can be found almost all over the body, and disruption of sleep-wake homeostasis.
What happens now?
A necessary next step is to continue to examine carefully whether the cancer risk of night work is confirmed in new epidemiological studies. For this purpose, a unified classification of shift work and the definition of relevant interruptions of the 24-hour rhythm (circadian disruption/ chronodisruption) has to be worked out, for the danger of misclassification and distorted results is high, as the past has shown. The term “shift work” is very complex and is performed worldwide in many varying environments. Here too, in studies only the most similar loads should be compared. What’s more it is not only external factors which play a role in the tolerance to night work, but also individual prerequisites for it, such as the chronotype. These could be queried to some degree in future studies.
Shift work recognised as an occupational disease
In Denmark until the end of 2011, breast cancer was recognised as an occupational disease for more than 100 women where these women had, among other things, been working for more than 20 years at least four nights a month. Nonetheless, this does not mean that for those affected the cause of the disease had been proven to be shift work. Rather other competing causes had to be excluded. This procedure is not possible in Germany, since in its vice-a-versa way, it is the causation which must be clearly demonstrated. The social security systems of the two countries differ too radically in order to make simple comparisons. In Denmark, all treatment costs are covered by national health insurance and only small pensions are paid out with occupational disease. Therefore, the requirements for recognition as an occupational disease are governed much more generously.
What can one do today?
Employers can increasingly optimise shift work according to the latest labour-physiological findings. For example, too many night shifts in a row should be avoided. The return adjustment to the normal day-night cycle for the body is usually after having worked through several nights (> 3 days) more onerous than the previous adjustment to the night shift itself. Sufficient recovery between shifts is also recommended as is participation of employees in design of a roster, and regular meals during the night work, to name just a few considerations. All this, last but not least, is nevertheless, in regard to the already certain health disorders created via disruption of the circadian rhythm, sensible action. In addition to limited daily performance, and short-term disturbances of the sleep-wake cycle and of the immune system, such disorders include first and foremost cardiovascular and gastrointestinal diseases.
Nothing has been proven
In animal experiments, there is impressive evidence that the disruption of circadian rhythms can cause cancer. The epidemiology can support this evidence but cannot unambiguously show this risk for humans so far. Possible formation pathways are known, but the relationship is too complex to conclusively evaluate its relevance in “normal life”. Scientists will be able to identify clear causalities in the foreseeable future only with difficulty. An optimisation of the working conditions of night workers is, however, both in terms of the known health problems, as well as of possible risk of cancer, to be recommended at all costs.