SummaryCircadian rhythms have been documented for various physiological variables, such as body temperature, heart rate, blood pressure, Cortisol, adrenaline (epinephrine) and noradrenaline (norepinephrine) under resting, as well as exercise conditions. The extent to which psychological variables, such as perception of effort, mood states and anxiety, are subject to circadian rhythms at rest and during exercise, is not as well understood.Body temperature follows a circadian rhythm in which temperature is highest in the evening and lowest in the early morning. Exercise heart rate follows a similar rhythm, but peaks a little earlier. The findings for exercise blood pressure are not consistent. Performance of physical activity is generally improved in the afternoon or evening, compared with morning. Cortisol levels follow a circadian rhythm in which Cortisol peaks in the morning and decreases throughout the rest of the day, at rest and during exercise. Plasma catecholamines show a rhythm at rest, but the findings during exercise are inconsistent.Research on circadian rhythms of perceived exertion and mood states are equivocal and the limited research on anxiety suggests that the anxiolytic response to exercise is not influenced by time of day.