I was watching a very well-done documentary about light pollution
The City Dark, that mentioned some off the recent work on the association between light pollution and breast cancer.
This study examines how blood from humans exposed to light at night is depleted of malatonin, and it helped breast cancers in rats to grow:
The increased breast cancer risk in female night shift workers has been
postulated to result from the suppression of pineal
melatonin production by exposure to light at night.
Exposure of rats bearing rat hepatomas or human breast cancer
xenografts
to increasing intensities of white fluorescent
light during each 12-hour dark phase (0-345 μW/cm2) resulted
in a dose-dependent suppression of nocturnal melatonin blood levels and a
stimulation of tumor growth and linoleic
acid uptake/metabolism to the mitogenic molecule
13-hydroxyoctadecadienoic acid. Venous blood samples were collected from
healthy, premenopausal female volunteers during
either the daytime, nighttime, or nighttime following 90 minutes of
ocular
bright, white fluorescent light exposure at 580
μW/cm2 (i.e., 2,800 lx). Compared with tumors perfused with
daytime-collected melatonin-deficient blood, human breast cancer
xenografts
and rat hepatomas perfused in situ, with
nocturnal, physiologically melatonin-rich blood collected during the
night, exhibited markedly suppressed proliferative
activity and linoleic acid uptake/metabolism.
Tumors perfused with melatonin-deficient blood collected following
ocular exposure
to light at night exhibited the daytime pattern of
high tumor proliferative activity. These results are the first to show
that the tumor growth response to exposure to light
during darkness is intensity dependent and that the human nocturnal,
circadian
melatonin signal not only inhibits human breast
cancer growth but that this effect is extinguished by short-term ocular
exposure
to bright, white light at night. These mechanistic
studies are the first to provide a rational biological explanation for
the increased breast cancer risk in female night
shift workers.
There are numerous studies now that show these connections. I am increasingly sympathetic to dark sky ordinances as a public health measure.
Another study suggests a connection to heart disease:
Background The purpose of this study was to examine prospectively the relation of shift work to risk of coronary heart disease (CHD)
in a cohort of women.
Methods and Results An ongoing
prospective cohort of US female nurses, in whom we assessed (in 1988)
the total number of years during which they
worked rotating night shifts (at least three
nights per month in addition to day and evening shifts), included 79 109
women,
42 to 67 years old in 1988, who were free of
diagnosed CHD and stroke. Incident CHD was defined as nonfatal
myocardial infarction
and fatal CHD. During 4 years of follow-up (1988
to 1992), 292 cases of incident CHD (248 nonfatal myocardial infarction
and
44 fatal CHD) occurred. The age-adjusted
relative risk of CHD was 1.38 (95% CI, 1.08 to 1.76) in women who
reported ever doing
shift work compared with those who had never
done so. The excess risk persisted after adjustment for cigarette
smoking and
a variety of other cardiovascular risk factors.
Compared with women who had never done shift work, the multivariate
adjusted
relative risks of CHD were 1.21 (95% CI, 0.92 to
1.59) among women reporting less than 6 years and 1.51 (95% CI, 1.12 to
2.03)
among those reporting 6 or more years of
rotating night shifts.
Conclusions These data are compatible with the possibility that 6 or more years of shift work may increase the risk of CHD in women.
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