Health Consequences of Circadian Disruption in Humans and Animal Models - Progress in Molecular Biology and Translational Science - page 291

the tau mutant hamster. Recently, it has been demonstrated that genetic dis-
ruption of the fly circadian clock through a per null mutation likewise
increases mortality and leads to accelerated aging. 86

4.1.3 SCN lesions
SCN lesions lead to arrhythmia at the overt level and if a lack of circadian
rhythmicity is detrimental to health, then one would expect SCN lesions to
decrease lifespan. While most studies have failed to find any pronounced
effects (e.g., see Refs. 87,88 ), it is important to note that most laboratory
conditions lack the rich ecological environment under which animals need
to acquire food and avoid becoming food. Within the few reports where
these variables are taken into account in seminaturalistic conditions, it has
been reported that arrhythmic animals with SCN lesions display a decreased
survival due to an increased predation risk 89 or some unspecified cause. 90
However, a recent study of mice with a per2 null mutation held in semi-
naturalistic enclosures did not find long-term negative effects on fitness or
survival. 91

4.2. Cancer and oxidative stress
Recent studies indicate use of artificial light at night contributes to increasing
rates of breast cancer in sighted women, 92-94 which is not evident in blind
women. 95 Furthermore, shift work and/or frequent air travel is associated
with higher risk for several forms of cancer, 96 including endometrial, 97
colorectal, 98 lymphatic, 99 prostate, 100-103 and breast cancer (reviewed in
Ref. 104 ). Moreover, shift workers present epigenetic profiles indicative
of an increased risk for tumorogenesis. 105,106 Melatonin suppression and/or
the disruption of its proper timing under shift work conditions may be an
important element in the increased risk of cancer development. 104,107 These
epidemiological findings have prompted the decision to reclassify shift work
from a possible to a probable human carcinogen (class 2A, International
Agency for Research on Cancer) and to compensate Danish shift workers
who developed breast cancer. 108

4.2.1 Environmental models
Nocturnal rodents provided a fast-rotating simulated jetlag schedule display
accelerated tumor growth relative to rodents that remain on static LD
cycles. 109-112 Increased tumor growth is likewise observed under static
LD cycles following SCN lesion. 113 Moreover, constant light increases
the rate of tumor growth in rodent model s 114-119

and adversely affects