Our circadian world shapes much of metabolic physiology. In mice ∼40% of the light and ∼80% of the dark phase time is characterized by bouts of increased energy expenditure (EE). These ultradian bouts have a higher body temperature (Tb) and thermal conductance and contain virtually all of the physical activity and awake time. Bout status is a better classifier of mouse physiology than photoperiod, with ultradian bouts superimposed on top of the circadian light/dark cycle. We suggest that the primary driver of ultradian bouts is a brain-initiated transition to a higher defended Tb of the active/awake state. Increased energy expenditure from brown adipose tissue, physical activity, and cardiac work combine to raise Tb from the lower defended Tb of the resting/sleeping state. Thus, unlike humans, much of mouse metabolic physiology is episodic with large ultradian increases in EE and Tb that correlate with the active/awake state and are poorly aligned with circadian cycling.

Diabetes Endocrinology and Obesity Branch National Institute of Diabetes and Digestive and Kidney Diseases NIH Bethesda MD 20892 USA

Diabetes Endocrinology and Obesity Branch National Institute of Diabetes and Digestive and Kidney Diseases NIH Bethesda MD 20892 USA; Centre for Experimental Medicine Institute for Clinical and Experimental Medicine Prague Czech Republic; Department of Biochemistry and Microbiology University of Chemistry and Technology Prague Czech Republic

Laboratory of Biological Modeling National Institute of Diabetes and Digestive and Kidney Diseases NIH Bethesda MD 20892 USA

Mouse Metabolism Core National Institute of Diabetes and Digestive and Kidney Diseases NIH Bethesda MD 20892 USA

Phoenix Epidemiology and Clinical Research Branch National Institute of Diabetes and Digestive and Kidney Diseases NIH Phoenix AZ 85016 USA

Phoenix Epidemiology and Clinical Research Branch National Institute of Diabetes and Digestive and Kidney Diseases NIH Phoenix AZ 85016 USA; Department of Information Engineering University of Pisa Pisa 56122 Italy

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