In 2007, the International Agency for Research
on Cancer (IARC) classified shift work which
involves disruption of circadian rhythms
as probably carcinogenic to humans. Since
then, among the possible effects of shift work
on health the potential association with cancer,
especially breast cancer, has gained particular
public attention.
Results from two large US prospective
cohort studies of a total of about 200,000
nurses are of special interest; among those
nurses who worked rotating night shifts for
many years more breast cancer cases were
observed than expected. Although in these
studies many potential confounders were
taken into account, other occupational risk
factors such as X-ray exposure or handling of
chemotherapeutics (which could not be addressed)
might have affected the observed
associations. Any interpretation of the results
of experimental studies in animals with
respect to a potential carcinogenic effect of
shift work in humans needs to consider that
exposure conditions such as constant light
exposure or constant darkness are not an adequate
correlate of shift work involving work
at night. There are theoretical considerations
of possible mechanisms, but it has not yet
been demonstrated that the mechanisms discussed
to date do indeed lead to cancer and
are relevant for humans. For example, the
question as to whether specific shift schedules
cause continuous reductions in melatonin
concentrations which then lead to raised oestrogen
levels favouring the development of
breast cancer – as one of the theories postulates
– has not yet been adequately studied.
To date, there is not sufficient evidence
for a carcinogenic effect of shift work (especially
night work). Large prospective cohort
studies involving different occupational
groups and precise assessment of shift work
exposure and covariates such as sleep quality
and lifestyle are required for more insight
into the relationship between shift work and
cancer. In addition, cell culture experiments
and animal studies in which the exposures
optimally reflect human shift work may help
to explore underlying mechanisms.
Most importantly, prevention requires
the identification of those shift schedules
with the least adverse effects on health. As
a further goal, the non-avoidable disturbing
influences of shift work on circadian rhythms
should be minimized.
