The International Agency for Research on Cancer (IARC) looked at shift work as part of their Monographs programme in October 2007. The result was published as Monograph 98 in 2010 and as a summary is available in Lancet Oncology.
The key conclusion is:
"On the basis of “limited evidence in humans for the carcinogenicity of shift-work that involves nightwork”, and “sufficient evidence in experimental animals for the carcinogenicity of light during the daily dark period (biological night)”, the Working Group concluded that “shift-work that involves circadian disruption is probably carcinogenic to humans” (Group 2A)."
Thus, compared to EMFs:
- the human evidence (mainly epidemiology) is classified the same - "limited"
- the animal evidence is classified as two categories stronger - "sufficient" instead of "inadequate"
- and this means the overall classification is stronger - "probably (2A)" instead of "possibly (2B)"
So the key difference is the animal evidence. For shift work, IARC found:
"More than 20 studies investigated the effect of constant light, dim light at night, simulated chronic jet lag, or circadian timing of carcinogens, and most showed a major increase in tumour incidence."
By contrast, there are no consistent and robust studies from animals showing increases in tumours for exposure to EMFs.
If shift work is a cause of cancer, the mechanism is almost certainly disruption of the circadian rhythm leading to various effects including suppression of melotinin production:
"Exposure to light at night disturbs the circadian system with alterations of sleep-activity patterns, suppression of melatonin production, and deregulation of circadian genes involved in cancer-related pathways. Inactivation of the circadian Period gene, Per2, promotes tumour development in mice, and in human breast and endometrial tumours, the expression of PERIOD genes is inhibited. In animals, melatonin suppression can lead to changes in the gonadotrophin axis. In humans, sleep deprivation and the ensuing melatonin suppression lead to immunodeficiency. For example, sleep deprivation suppresses natural killer-cell activity and changes the T-helper 1/T-helper 2 cytokine balance, reducing cellular immune defence and surveillance."
It is sometimes suggested that disruption of circadian rhythms or of melatonin levels may be a mechanism for magnetic fields to affect humans as well, though this is not established.