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system, 37 misalignment among neural oscillators within the SCN, 37-39 and
misalignment among molecular components of the circadian clock. 40 In
addition, a subset of Siberian hamsters provided a specific jetlag protocol will
not resynchronize and, instead, will remain perpetually misaligned or
become arrhythmic. 41 Unusual resetting behavior has also been reported
to occur in human s 42 and diurnal rodent species. 43,44 These forms of
odd resetting behavior may reflect that the circadian clock has been arrested
by light occurring during mid-subjective night, a time in the circadian cycle
that is predicted to be a phase singularity point. 45
3.2.3 Non-24 h LD cycles
In the laboratory, misalignment between internal timing and external cycles
can be produced through exposure to non-24 h LD cycles, 46,47 which may
be similar to work schedules commonly employed in some occupations. 48
As the circadian system is able to entrain to non-24 h cycles only within a
certain range (typically 24
2 h), exposure to non-24 h LD cycles outside
these limits causes rhythms to “free-run,” with intermittent and temporary
alignment between the internal and external milieu. In humans, exposure to
non-24 h LD cycles can also cause internal desynchrony, where different
rhythms free-run independently of one another (e.g., body temperature,
melatonin, and sleep/wake rhythms). 49 Lastly, exposing rodents to non-
24 h LD cycles can cause different SCN compartments to desynchronize
such that one is synchronized to the external lighting environment and
the other free-runs. 50
3.2.4 Light at night
Exposure to light at night is a feature of environmental conditions found in
both ecological and industrialized settings. Annual fluctuations in day length
entail changes in duration of daytime light exposure, although use of artifi-
cial light in humans can serve to attenuate seasonal changes. That even low
levels of light at night can reset circadian phase and suppress melatonin
release in humans 6,7 suggests that the relatively common exposure to artifi-
cial light may in fact affect circadian function. One classic circadian manip-
ulation is exposure to constant light conditions, which can produce internal
desynchrony in humans. 51 In nocturnal rodents, exposure to constant light
lengthens the circadian period and can cause arrhythmia or “splitting” of
behavioral and physiological rhythms. 52,53 Recent work has also investi-
gated the effects of light pollution at night by providing nocturnal rodents
with low levels of light at night (typically
5 lux) under an otherwise normal
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