Physical Exercise : A Countermeasure Against Disruptions of the Circadian Timing System During Spaceflight?

A growing body of research indicates that any de-synchronization of the circadian Timing system (CTS) can be detrimental to mental and physical health [1]. It has also been reported that space mission can induce a misalignment of the CTS, and substantially impact crew health and safety [2]. Physical exercise on the other hand has recently been outlined as a potential countermeasure to promote the entrainment of the CTS. Previous studies investigating the role of exercise on the CTS have, however, often lacked strictly controlled laboratory conditions isolating the effects that can be purely attributed to exercise. Bedrest offers an excellent model not only for simulating fluid shifts observed during micro-g exposure, but also exposes subjects to semi-isolation and confinement while allowing to strictly control various potential confounding effects. The aim of the present study was therefore (1) To investigate the effect of 60 days of bed-rest on circadian rhythm. (2) To investigate if any changes in circadian rhythm can be compensated by resistance exercise (RES) or resistance exercises superimposed by whole-body vibrations (RVE). 24 healthy men were exposed to 60 days of 6-degree head-down tilt bed-rest as part of the Berlin Bed-rest Study 2 (BBR2-2). Subjects were randomly allocated to a resistance (RES), a combined resistance and vibration exercise (RVE), or a control (CTR) group. Core body temperature profiles for 36 h (7.30 p.m. to +1d 6.30 a.m.) were determined at week 1 and week 7 using a non-invasive heatflux sensor (Double Sensor) positioned at the forehead. Cosinor analysis was employed to quantify circadian rhythm by mesor, acrophase, and amplitude. Bed-rest induced a phase delay of 45 min in CTR (P = 0.051). This delay was offset by both exercise groups, indicating that exercise may also induce a phase advance. In addition, CTR showed more variation in circadian rhythmicity compared to RVE and RE. The effect of exercise on advancing phase confirms previous findings [3,4], supporting the notion that exercise may potentially offset disruptions of circadian rhythm. Future studies should therefore consider investigating the role of exercise as a countermeasure when circadian rhythm is prone to misalignments during long-term spaceflight.