It is the great forgotten and regretted of the pension reform which has angered the French for several weeks: while the text plans to shift the minimum age of departure to 64 years, it says almost nothing about the hardship at work . Which makes Martine’s victory, after two years of fighting, even more symbolic. In just over 25 years of practice, this ex-nurse has spent more than 850 nights in hospital. Hundreds of hours of night work, between 1982 and 2009, until the announcement of breast cancer. Supported by the local section of the CFDT-Minors, she managed, last January, to have her tumor recognized as an occupational disease.
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Because some scientists have been interested for several decades in the health risks incurred by workers subjected to precarious working conditions. “The case of this nurse is therefore a culmination, a big step, but also the beginning of other work » rejoices Émilie Cordina, research engineer at Inserm,National Institute of Health and Medical Research. Gradually, the scientific consensus is emerging: night work, itself increasingly widespread, increases the risk of breast cancer.
Long time work
“In the 1990s, American researchers showed for the first time, on a cohort of nurses, an association between breast cancer and night hours”, explains Pascal Guénel, research director at Inserm and researcher in epidemiology. This publication lays the first stone: other studies follow, in different countries. But establishing a formal correlation is complex, if only to define what night work is: what hours are we talking about? How regularly?
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The team led by Pascal Guénel, to which Émilie Cordina belongs, thus published in 2018 a study bringing together five other works by taking precise criteria. They then define night work as an exercise ” at least three hours between midnight and 5 a.m.”. Their resultswhich include more than 12,000 patients worldwide, are formal. “Among premenopausal women, night work increases the risk of breast cancer by 26%”. And this one “seems particularly to increase among women who have worked more than two nights a week for more than ten years”, but decreases after stopping night work, the authors continue. It remains to be seen why the risk no longer increases after menopause, “which could be explained by the fact that after a certain age, we work less at night”, says Pascal Guénel. Even better in 2019: the International Agency for Research on Cancer (IARC) then renews its classification of night work as “probably carcinogenic”. “This work takes up all the literature on night work and cancer, not just the breast”, explains Pascal Guénel, who participated in this international work. Besides breast cancer, la classification relates to prostate cancer.
Disrupted biological clock
Why these types of tumors and not others? Already, because they are the most studied, because they are also the most frequent. But also because they are often hormone-dependent, that is to say that their growth is favored by certain hormones. Gold, “not sleeping at the right hours affects the production of hormones”, explains Emilie Cordina. And more specifically, one of them: melatonin. “At night, we have a spike in melatonin, a hormone produced by a gland in the brain”, specifies in turn Pascal Guénel. And it decreases when exposed to light, so when you wake up.
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But precisely, in night workers, this peak of melatonin, which plays on our biological clock, or circadian rhythm, does not occur. “But the latter very probably has anti-cancer effects and plays on the production of other hormones”, continues the epidemiologist. A disturbance of our internal ticking that could explain how having a night job makes us more likely to develop this pathology. “But often, the causal link is particularly complex to establish, there are several factors that come into play during the development of cancer”, tempers Emilie Cordina. The consumption of alcohol and tobacco, often more widespread in this population, could for example play a role. “And sleep is delayed, but also often shortened, which increases the risk of other biological disorders and inflammations”, adds Pascal Guénel. Nocturnal exercise notably promotes sleep and mood disorders, obesity, cardiovascular diseases, etc. Which can themselves play on the development of a tumour.
Million workers
This complexity in defining the underlying mechanisms and deciding, in each patient, on the responsibility for their working conditions, makes the broader recognition of tumors as an occupational disease complex. Because if studies now show that night work increases the risk of breast cancer in a population, establishing causality on a case-by-case basis is more complex: in fact, breast cancer linked to night work does not belong to the health insurance table of occupational diseases, each situation must be examined individually to establish the responsibility of the schedules in the appearance of the disease. So that the scientific consensus on the increase in this risk does not mean that all people who work after sunset will benefit from the recognition of occupational disease if they face this disease – unless it was added to the CPAM list. “But our job is also to determine the types of organization of night work that minimize the risks”, explains Pascal Guénel. Are the health consequences reduced when taking a nap? Or if the person works less than three nights a week? This knowledge could, in the long term, make it possible to establish recommendations.
And the guidelines that emerge from this research could affect millions of workers. In recent decades, thee number of employees working at night has continued to increase in France: from 3.3 million, or 15% of working people, in 1990, they are now at 4.3 million, or 16.3%, in 2013. It concerns above all nurses, midwives and nurses’ aides, security guards, the army, police and firefighters and road and delivery drivers. According to the World Health Organization (WHO), globally, it is estimated that one in five workers is engaged in regular night work, with percentages tending to increase over time in some countries. Why impose this subject on the government’s agenda?