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feeding period is the size of the preceding meal. Indeed, larger meals lead to
certainty whether these ultradian patterns of intermeal lags are disturbed in
clock mutant or mice knockout for clock genes.
Furthermore, in addition to the circadian and homeostatic control, food
intake can be influenced by the ambient lighting conditions, defining direct
effects of light or “masking.” In nocturnal mammals, light exposure at night
acutely reduces food intake during the active period (i.e., negative masking
of feeding), while dark exposure during daytime enhances food intake dur-
ing the usual sleep/fasting period (i.e., positive masking of feeding).
31
2.2. Daily variations in energy metabolism
Mammals maintain a relatively high metabolic rate with narrow daily var-
iations, albeit they do not feed continuously. Nonetheless, it is possible to
measure with indirect calorimetry daily oscillations in energy expenditure
(via oxygen consumption) and in respiratory exchange ratio, also called
respiratory quotient (RQ; i.e., the ratio of carbon dioxide produced and
oxygen consumed), which is an indicator of metabolized fuels. Lesions of
the suprachiasmatic clock suppress circadian rhythmicity of energy expen-
that the master clock controls the daily variations in energy metabolism.
Alternatively, this arrhythmicity may result from the arrhythmic sleep-wake
cycle that would prevent the detection of circadian variations.
When mammals are food deprived for several days under a light-dark
cycle, daily rhythms of energy metabolites persist, thus demonstrating that
the metabolic rhythmicity does not rely solely on daily feeding-fasting
metabolites and RQ as well.
33,34
Per2
-null mice have been found to be leaner due to increased energy
expenditure, while 24-h RQ values do not differ from wild-type litter-
mates.
35
Clock
mutant mice are less active during nighttime compared to
wild-type mice and, accordingly, display a reduction in nocturnal energy
nal rhythm of energy expenditure is dampened, due to reduced expenditure
variations of the RQ show large interindividual differences in
Cry1
/
;
leads to an altered daily rhythm of
Cry2
/
in vivo
carbohydrate/lipid utilization, as
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