[Circadian health at the crossroads of physiology and behavior]

Circadian rhythms are major determinants of physical and mental health and sleep at the crossroads of physiology and behavior. Circadian physiology is traditionally described according to three main dimensions: circadian phase, amplitude, and stability. Nycthemeral behaviors are typically characterized by two primary dimensions: nycthemeral timing and regularity. Circadian-related sleep complaints arise from a circadian disruption between circadian physiology and nycthemeral behaviors and represent a potential sixth clinical dimension. Based on scientific knowledge, this article defines each of these dimensions, explores their main determinants (genetics, age and sex, external factors), and presents objective (melatonin, temperature, actimetry) and subjective (sleep diaries, self-report questionnaires) measurement tools. The interactions between these six dimensions are discussed, along with their impact on sleep and health (mental, cardiovascular, metabolic, immune health), and on physical and cognitive performance. Finally, a definition of circadian health is proposed, as well as strategies for its promotion (light exposure, physical activity, melatonin), offering new opportunities for improving public health in both general and clinical populations. Promoting circadian health should take into account all external factors (individual, social, societal, environmental) to be effective and contribute to reducing health inequalities. In the future, developing a unique tool to measure circadian health could enhance its characterization and management in both general and clinical populations.