Abstract: The invention relates to a method for determining a circadian rhythm of a subject, comprising providing a sample from a subject, such as a hair root, oral mucosa, or a saliva sample, quantifying RNA expression levels of circadian phase genes and amplitude prediction genes in said sample, and applying the expression levels in a mathematical model. The invention further relates to diagnostic and/or medical uses of the invention, and corresponding therapeutic methods, comprising provision of optimal and/or recommended physical activity times based on the circadian rhythm, such as times recommended for sleeping, working, meal intake, sport, intellectual tasks, light exposure, medical intervention, and/or medical diagnostics. In further aspects, the invention relates to a computer-readable storage medium adapted to configure a processor to perform a method comprising executing a biomathematical model algorithm based on the method of the invention.