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Highlights d Thyroid hormone induces browning independent of sympathetic nervous system through TRb d Despite high UCP1, the beige fat lacks adrenergic input and is inactive d Thyroid hormone's metabolic and thermogenic effects are maintained in UCP1 knockout mice d Thyroid hormone induces hyperthermia and elevates the body temperature setpoint

24-h rhythms in physiology and behaviour are organized by a body-wide network of endogenous circadian clocks. In mammals, a central pacemaker in the hypothalamic suprachiasmatic nucleus (SCN) integrates external light information to adapt cellular clocks in all tissues and organs to the external light-dark cycle. Together, central and peripheral clocks co-regulate physiological rhythms and functions. In this review, we outline the current knowledge about the routes of communication between the environment, the main pacemakers and the downstream clocks in the body, focusing on what we currently know and what we still need to understand about the communication mechanisms by which centrally and peripherally controlled timing signals coordinate physiological functions and behaviour. We highlight recent findings that shed new light on the internal organization and function of the SCN and neuroendocrine mechanisms mediating clock-to-clock coupling. These findings have implications for our understanding of circadian network entrainment and for potential manipulations of the circadian clock system in therapeutic settings.

Endogenous circadian clocks regulate 24-h rhythms of behavior and physiology to align with external time. The endocrine system serves as a major clock output to regulate various biological processes. Recent findings suggest that some of the rhythmic hormones can also provide feedback to the circadian system at various levels, thus contributing to maintaining the robustness of endogenous rhythmicity. This delicate balance of clock-hormone interaction is vulnerable to modern lifestyle factors such as shiftwork or high-calorie diets, altering physiological set points. In this review, we summarize the current knowledge on the communication between the circadian timing and endocrine systems, with a focus on adrenal glucocorticoids and metabolic peptide hormones. We explore the potential role of hormones as systemic feedback signals to adjust clock function and their relevance for the maintenance of physiological and metabolic circadian homeostasis.

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