Zeitgeber-effects of exercise on human circadian rhythms: what are alternative approaches to investigating the existence of a phase-response curve to exercise?

Abstract

Over the last decade, there has been some evidence reported on humans from both field-based and laboratory-based research that exercise exerts phase-shifting effects on the body clock. Following a recently published update on whether exercise is a significant synchroniser of human circadian rhythms, the aims of this review are to consider the possibility that exercise acts as a zeitgeber in accord with a phase response curve, and suggest applications of this knowledge to investigate further rhythm disturbances such as shift-work and jet lag. The application of exercise as a treatment in the field for rhythm disruption (for example, after jet lag and when there is shift-work intolerance) has been hampered somewhat from lack of control of competing zeitgebers (e.g. light), exercise characteristics (such as mode, intensity and duration), and athletic status of the research participants (from sedentary individuals to Olympic athletes). Standardisation of these factors is needed for evidence-based advice of a practical nature to be provided to travellers or shift-workers. Data from laboratory-based experiments completed in the last 10 years have also been affected by lack of control of the above factors and have the additional problem of how best to control for the pronounced direct (masking) effects of exercise. Nevertheless, it is now known that exercise bouts of quite varied durations and intensities can mediate phase shifts, independently from those produced by light, in populations heterogeneous in terms of fitness and age. More research is still needed to elucidate more fully (i) a phase response curve for exercise that is robust and does not depend on the type of marker used for assessing phase (secretory products or body temperature) and (ii) the mechanisms producing any phase shifts.

Keywords:

• physical activity
• masking
• rhythm entrainment
• exercise intensity
• exercise duration