Health Consequences of Circadian Disruption in Humans and Animal Models - Progress in Molecular Biology and Translational Science - page 284

1. INTRODUCTION
Modern times demand much of the circadian system that coordinates
myriad rhythms in our bodies. Nontraditional work schedules are becoming
increasingly commonplace with the demand for goods and services at all
hours of the day, rapid industrialization, and economic globalization. Sched-
ules that include night shifts require employees to be awake during night-
time hours when their biological clocks program inactivity. Mounting
evidence indicates that shift work is associated with a wide variety of serious
physiological and psychological disorders, including cardiovascular disease,
metabolic syndrome, obesity, immune dysfunction, increased risk for can-
cer, and reproductive complications. In fact, epidemiological studies may
underestimate the adverse health consequences of shift work owing to
self-selection and survival bias.
The three main factors thought to produce adverse effects on perfor-
mance and health in shift workers are (1) circadian misalignment, (2) sleep
deprivation, and (3) exposure to light at night. However, the specific role of
circadian disruption can be difficult to assess in epidemiological reports
because of variability in shift exposure conditions and the potential contri-
bution of covariates (i.e., changes in diet, social stress, and the use of tobacco,
alcohol, and prescription drugs). Epidemiological reports that control for
some of these noncircadian variables do find effects of circadian disruption.
Together with experimental evidence, these reports strongly indicate that
circadian disruption and misalignment are key factors contributing to the
adverse health consequences commonly observed in shift workers. Studies
with animal models of circadian disruption are useful in that they afford stri-
cter control to investigate the contribution of circadian misalignment in iso-
lation of, or in combination with, these other factors.
This review summarizes the pervasive effects of circadian disruption on
health in humans and animal models. First, we describe the circadian system
and types of circadian disruption in humans along with the corresponding
experimental manipulations used to simulate these conditions in humans
and animal models. Second, we provide a comprehensive review of the
health consequences that are evident under these conditions and discuss
the factors that are thought to contribute to these disease states. Finally,
we discuss future areas of research that can identify the specific causes and
consequences, as well as the countermeasures for the health consequences
of circadian disruption.