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

Circadian rhythms in bed rest: : Monitoring core body temperature via heat-flux approach is superior to skin surface temperature

Continuous recordings of core body temperature (CBT) are a well-established approach in describing circadian rhythms. Given the discomfort of invasive CBT measurement techniques, the use of skin temperature recordings has been proposed as a surrogate. More recently, we proposed a heat-flux approach (the so-called Double Sensor) for monitoring CBT. Studies investigating the reliability of the heat-flux approach over a 24-hour period, as well as comparisons with skin temperature recordings, are however lacking. The first aim of the study was therefore to compare rectal, skin, and heat-flux temperature recordings for monitoring circadian rhythm. In addition, to assess the optimal placement of sensor probes, we also investigated the effect of different anatomical measurement sites, i.e. sensor probes positioned at the forehead vs. the sternum. Data were collected as part of the Berlin BedRest study (BBR2-2) under controlled, standardized, and thermoneutral conditions. 24-hours temperature data of seven healthy males were collected after 50 days of -6\ufffd head-down tilt bed-rest. Mean Pearson correlation coefficients indicated a high association between rectal and forehead temperature recordings (r > 0.80 for skin and Double Sensor). In contrast, only a poor to moderate relationship was observed for sensors positioned at the sternum (r = -0.02 and r = 0.52 for skin and Double Sensor, respectively). Cross-correlation analyses further confirmed the feasibility of the forehead as a preferred monitoring site. The phase difference between forehead Double Sensor and rectal recordings was not statistically different from zero (p = 0.313), and was significantly smaller than the phase difference between forehead skin and rectal temperatures (p = 0.016). These findings were substantiated by cosinor analyses, revealing significant differences for mesor, amplitude, and acrophase between rectal and forehead skin temperature recordings, but not between forehead Double Sensor and rectal temperature measurements. Finally, Bland-Altman analysis indicated narrower limits of agreement for rhythm parameters between rectal and Double Sensor measurements compared to between rectal and skin recordings, irrespective of the measurement site (i.e. forehead, sternum). Based on these data we conclude that (1) Double Sensor recordings are significantly superior to skin temperature measurements for non-invasively assessing the circadian rhythm of rectal temperature, and (2) temperature rhythms from the sternum are less reliable than from the forehead. We suggest that forehead Double Sensor recordings may provide a surrogate for rectal temperature in circadian rhythm research, where constant routine protocols are applied. Future studies will be needed to assess the sensor\ufffds ecological validity outside the laboratory under changing environmental and physiological conditions

Monitoring desynchronization of the circadian timing system in space and during isolation and confinement

Given the increasing duration of space flights problems associated with changes of the circadian rhythm might be increasingly prominent during future exploratory missions. Therefore, technological advances for simple and accurate monitoring of circadian rhythm are clearly needed. Furthermore, such technology would provide the basis for countermeasures and direct treatment for proper reentrainment and synchronization of the circadian system to maintain proper physical and mental wellbeing and performance. In the present study it was shown that during 50 days of bed-rest no changes in circadian rhythm can be observed. It is suggested that both strength and exposure to external 'Zeitgeber' during bed-rest account for these findings. First, all subjects were exposed to various constant social routines such as strict wake-up schedules, regular meals and experimental schedules. In particular, the presence of changing light and dark cycles during the entire study is likely to explain the lack of any significant changes in circadian rhythm. Due to the small sample size, however, these results need to be interpreted with caution. These findings are well in accordance with research from Antarctic missions, stressing the importance of photonic cues for the entrainment of circadian rhythm. The second aim of the present study was to investigate the accuracy of determining circadian rhythm of core body temperature using a new, non-invasive heatflux sensor located at the head (Double Sensor) compared to rectal temperature measurements. The second aim of the present study was to investigate the accuracy of determining circadian rhythm using a new, non-invasive heatflux sensor located at the head (Double Sensor) based temperature recordings compared to rectal temperature measurements. No significant differences could be found for acrophase, mesor, and amplitude. This confirms our previous findings and we conclude that temperature profiles using the Double Sensor technology is well suitable for determining circadian rhythm under thermoneutral conditions. In conclusion, the present results confirm the validity of the Double Sensor technology to determine circadian rhythm and support the implementation of this approach in future space missions to i) better understand the time course and basic principles of the adaptations pertaining to the human autonomic nervous system in space, ii) to adjust more adequately physical exercise as well as rest- and work shifts, and iii) to foster adequate workplace illumination in the sense of occupational healthcare for humans in space