Health Consequences of Circadian Disruption in Humans and Animal Models - Progress in Molecular Biology and Translational Science - page 286

gene/protein expression, influencing the pace and amplitude of circadian
rhythms (e.g., REV-ERB and ROR). Moreover, these transcriptional-
translational feedback loops interact with electrical and metabolic oscillations
to influence cellular function. 9,10
3. TYPES OF CIRCADIAN DISRUPTION IN HUMANS
AND ANIMAL MODELS
3.1. Circadian disruption in the real world
3.1.1 Nontraditional work schedules
Nontraditional work schedules are defined as those that occur outside the
hours of 9 am-5 pm, but this category encompasses a wide variety of work
schedules, including fixed night shifts, early morning shifts, flex shifts, ex-
tended shifts, rotating shifts, and frequent international travel. 11 It is estimated
that more than 15% of US employees ( > 20 million Americans) work on a
nontraditional schedule, 12-15 leading to a higher incidence of sick leave, stress,
decreased productivity, and accidents on and off the job with an estimated cost
exceeding $200 billion a year. 16,17 While shift workers can force themselves to
be awake at night and asleep during the day, this oftentimes entails a reduced
alertness and sleep efficiency. 18 Despite the ability tomeet expected behavioral
demands through conscious efforts, themajority of shift workers do not display
a corresponding shift in circadian function, and instead, remain in a statewhere
the biological clock is programming sleep during the nighttime hours and
wakefulness during the daytime hours. 19,20
3.1.2 Transmeridian travel
After transmeridian travel, our biological clocks gradually realign to the new
time zone, and the transient mismatch between internal timing and external
conditions produces symptoms of jetlag that might last for days to weeks. 21
While jetlag is often viewed as a temporary inconvenience, the widespread
and varied symptoms of jetlag are emblematic of the influence and impor-
tance of the circadian system for human health. Following transmeridian
travel, it typically requires 1 day to adjust for each time zone crossed and
the latency to recover is longer when traveling in the eastward
direction, 21 although the extent and severity of jetlag can be affected by a
host of personal and experiential factors. Moreover, large population surveys
indicate that many people are affected by “social jetlag,” a term that refers to
a misalignment between social demands and the inherently preferred time as