Circadian rhythms have been documented for various physiological varia
bles, such as body temperature, heart rate, blood pressure, cortisol,
adrenaline (epinephrine) and noradrenaline (norepinephrine) under rest
ing, 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 w
ell understood. Body temperature follows a circadian rhythm in which t
emperature is highest in the evening and lowest in the early morning.
Exercise heart rate follows a similar rhythm, but peaks a little earli
er. The findings for exercise blood pressure are not consistent. Perfo
rmance of physical activity is generally improved in the afternoon or
evening, compared with morning. Cortisol levels follow a circadian rhy
thm in which cortisol peaks in the morning and decreases throughout th
e rest of the day, at rest and during exercise. Plasma catecholamines
show a rhythm at rest, but the findings during exercise are inconsiste
nt. Research on circadian rhythms of perceived exertion and mood state
s are equivocal and the limited research on anxiety suggests that the
anxiolytic response to exercise is not influenced by time of day.