Verdensundhedsorganisationen WHO’s kræftforskningsinstitut i Lyon har vurderet den videnskabelige litteratur om sammenhæng mellem skiftarbejde (natarbejde) og kræft.

(Oktober 2007).

Nedenstående er ikke den endelige vurdering men et arbejdspapir som kun ventes ændret lidt i den endelige udgave.

 

 

• Shift-work: Types of shift systems were described, although from a biological perspective, it was assumed that the presence and amount of night work is the most important factor to be considered for the disruption of biological functions. Sleep is decreased in quantity and quality both on night shifts and on early morning shifts. In 2005, in Europe, shift work including night work involved more than 17% of the total working population, although large differences were found among countries (from 6.4- 30%).  Melatonin (easily measured in saliva) or its main urinary metabolite (6-sulfatoxymelatonin) could be a useful biomarker of circadian dysregulation.

Based on the underlying hormonal mechanism, risk of female breast cancer in cohorts of nurses and cabin crew was revised. There were limitations identified for studies in both groups of shift workers, due to uncontrolled confounding factors, bias on the detection of breast cancer due to higher prevalence of screening in these occupations, and, basically in the exposure assessments. Despite of these limitations, the working group concluded that the weight of evidence for breast cancer after shift work is rather convincing, though limited.

Under the same hypothesis, prostate, colon and endometrial cancers were also studied. Airline pilot studies showed markedly elevated prostate cancer incidence, but PSA follow-up did not allow a consistent interpretation of the findings.

Animal models have been used extensively for testing impact of the circadian system (central circadian pacemaker in the suprachiasmatic nuclei and the pineal gland/melatonin generating system) and its disruption (e.g. phase shifts, light contamination during darkness, melatonin and its suppression) on tumour development and growth at all phases of oncogenesis. A summary of significant positive studies (45/56) for each type of model and protocol in experimental studies of the effect of circadian rhythms on cancer incidence and growth, included: alterations in light exposures, SNC lesions, chronic experimental jet lag, pinealectomy-induced melatonin suppression, direct manipulation of melatonin, clock gene mutations and circadian timing of carcinogen administration. The relation of these experimental animal data and the shift work induced carcinogenesis were complemented with mechanistic considerations in relation to the disturbance of the circadian time organization due to light at night with alteration of the sleep-activity pattern, potential melatonin suppression and circadian gene alterations. The misalignment with the surrounding day-active social environment was also judged as an added factor.

Melatonin suppression also affects the gonadotropic axis, the immune system (prolactin), cell proliferation and scavenging of free radicals. Sleep deprivation favours insulin-resistance and obesity. Thus, both tumour induction and tumour promotion might be involved in the final effects seen.

 

The overall evaluation was "shift work (including night work) is probably carcinogenic to humans (Group 2A)".