Defining circadian health: Conceptual framework and development of a circadian health index

Circadian rhythms are key determinants of physical and mental health at the nexus of physiology and behavior. Classically, endogenous circadian rhythms are characterized according to three principal dimensions: circadian phase, amplitude and stability. From a behavioral perspective, the timing and regularity of nychthemeral behaviors represent two additional dimensions, and we propose as a sixth dimension the sleep complaints arising from a circadian disruption due to a mismatch between circadian physiology and nycthemeral behaviors. This article reviews each of these dimensions and examines their interactions, along with their effects on sleep and health. On this basis, we propose an integrated definition of circadian health. We then review both, objective (melatonin, temperature, actimetry) and subjective (sleep diaries, self-report questionnaires) tools for assessing each of the circadian health dimensions. Finally, we propose a novel tool aimed at assessing those circadian health dimensions as well as the computation of a composite index to quantify circadian health, along with a graphical representation to visualize it. While further validation is still needed, this proposal will help clinicians and researchers better decipher circadian rhythms and their impact on mental and physical health and may offer new opportunities for public health promotion in both general and clinical populations.