Hot take on a cool topic: is there really a need for a systemic master mediator of metabolic suppression in mouse daily torpor?

We argue in this perspective that daily torpor in the mouse, induced by a lack of caloric intake and a relatively cool environment, does not require a dedicated "master mediator" for systemic metabolic rate (MR) suppression. Instead, a set of centrally mediated changes, most notably: cessation of physical activity; dramatically reduced thermogenesis, along with a lowering of the defended minimum body temperature (Tb); and, reduced cardiovascular work, together drive the substantial drop in metabolic rate at the initiation of daily torpor in the mouse. That is, rather than invoking a global, organism-wide suppression of metabolism across all tissues, we argue that MR reduction during mouse daily torpor is selective and physiologically organized, targeting a small number of energetically dominant organ systems. Once Tb has declined in response to the initial physiological adaptations, temperature-dependent effects on biochemical reaction rates can account for much of the further reduction in whole-animal MR during the torpid state, resulting in an ultimate fall in MR of 75-80% in daily torpor of the mouse. This framework also aligns torpor with the normal ultradian rhythm of sleep-associated reductions in MR and Tb, differing mainly in the much deeper nadir reached.