The effect of the circadian and menstrual cycles on cardiovascular and thermoregulatory responses to exercise

Many physiological variables exhibit circadian rhythmicity. The circadian rhythm in core temperature is a well-established and it has been extensively studied during both passive and exercise heat exposure. In females, a circamensal rhythm in core temperature is also present and well established. However, there is little knowledge about whether there is an interaction effect between time of day and phase of menstrual cycle on core temperature and thermoregulatory and cardiovascular responses during and following exercise. The studies in the present thesis were designed to investigate such an interaction effect on the effector responses of the thermoregulation and cardiovascular systems during the exercise and post-exercise periods. The first experiment was designed to examine the time of day effect on thermoregulatory and cardiovascular responses during and following exercise in female subjects. Eight healthy participants completed 30-min exercise at 65%Y02peak at 07:00 and 19:00hr. Core temperature was significantly higher by 0.3, 0.4 and 0.3DC at rest (P=O.OOI), during the exercise (P=O.OOI) and post-exercise (P=0.008) periods in the evening compared to the morning. The second experiment was designed to examine the phases of menstrual cycle on thermoregulatory and cardiovascular responses during and following exercise. Ten healthy participants completed two exercise protocols (65%Y02peak) during the late follicular and luteal phases of the menstrual cycle (day 10-12 and day 20-22 after the onset of menstruation, respectively). Core temperature tended to be higher in the luteal phase compared with the late follicular phase (0.2DC) both at rest (p=O.064) and during exercise (p=0.062), whereas the heat-loss mechanisms were unaffected by menstrual cycle phase. In addition, resting stroke volume and cardiac output was greater in the late follicular phase compared to the luteal phase. The third experiment was designed to explore the interaction effect between time of day and phase of the menstrual cycle on thermoregulatory and cardiovascular responses during the exercise and the post-exercise periods. Ten healthy participants completed four exercise protocol (65% Y02peak); two exercise protocols in the morning of the late follicular and luteal phases, and two exercise protocols in the evening of the late follicular and luteal phases. Core temperature was higher in the evening of both phases of the menstrual cycle during exercise (p=O.OOI) and the post-exercise periods (p=O.003). There was an interaction effect between times of day and phase of the menstrual cycle on mean skin temperature during the exercise (p=0.048) and the post-exercise periods (p=0.006), a lower mean skin temperature in the evening compared with the morning during the late follicular phase and higher in the evening than in the morning in the luteal phase. However, there was no interaction of times of day and phase of menstrual cycle for other thermoregulatory and cardiovascular parameters measured. The results in the thesis indicate that temperature regulation is set around higher values in the evening and late luteal phase of the cycle, but that these changes are likely independent of each other. Future work, should more systematically investigate these responses, collecting data at more times of day and phases of the cycle.EThOS - Electronic Theses Online ServiceGBUnited Kingdo