Circadian Clocks, Food Intake, and Metabolism - Progress in Molecular Biology and Translational Science - page 120

during the light phase than mice exposed to regular light-dark cycle. 125
Also, mice exposed to constant light show larger gain in body gain and
changes in daily insulin sensitivity, as compared to individuals kept under
a light-dark cycle. 126
Disruption in circadian rhythmicity can be triggered by light exposure at
unusual times of the daily cycle. In humans, transmeridian fast travels (across
more than two or three time zones) have become very common following
the considerable development of air transport. Physiologically, they cause a
transient loss of circadian synchronization, internal regulations being initially
out of phase with respect to the new light-dark cycle. Then, day after day,
the suprachiasmatic clock will resynchronize to the new cycle and impose an
appropriate adjustment of peripheral clocks and oscillators to the local time.
The transient period of resynchronisation, relatively proportional to the
number of time zones crossed (but dependent on the east-west direction
of travel), is accompanied by sleep quality problems, digestive disorders,
and several metabolic and hormonal alterations. 127 In particular, carbohy-
drate oxidation was increased in human subjects exposed to 3 days of jet
lag, while protein oxidation was decreased. 128
Shift work and rotating work schedules trigger chronobiological con-
flicts between the endogenous clockwork and the ambient light environ-
ment as well as mealtime, leading to situations of altered internal
temporal organization (e.g., between the master clock and peripheral oscil-
lators or between different peripheral oscillators) and occurrences of
desynchronization (misalignment between internal timing and local time).
The deleterious effects of chronic desynchronization on metabolic health
have been identified in animals. Rats undergoing desynchronization caused
by a long-term biweekly change of the light-dark cycle are overweight or
have impaired insulin secretion compared to animals maintained in a fixed
light-dark cycle. 129,130 Moreover, rats forced to exercise during their usual
sleep period show a reversed rhythm of triacylglycerols and increased gain in
body mass. 131 Obesity and increased body mass index are commonly
observed in large-scale epidemiological studies on night workers and
workers with rotating schedules. 132-136 The obesogenic properties of
repeated light-dark shifts in animals or chronic shift work in humans leads
to the concept that we called “chronobesity.” 137
Mice exposed to light-dark cycles that are too short (i.e., 20 h) to allow
daily photic resetting of their master clock show cardiovascular disease,
larger gain in body mass, large insulin/glucose ratio, the latter indicating
insulin resistance. 138,139

In animal studies, ultimate desynchrony (i.e.,