Effect Of The Diurnal Variation In Core Temperature
Sebastien Racinais Ph.D. ASPETAR, Qatar Orthopedic and Sports Medicine Hospital, Exercise and Sports Science Department, Doha, Qatar
One of the most known circadian rhythms is core temperature. Human core temperature is generally measured by rectally and displays an acrophase in the late afternoon, close to 18:00 h (Waterhouse et al. 1993). This circadian rhythm is mainly endogenous and persists in free-living conditions, without influence of the zeitgebers (Colin et al. 1968).
Even if the absolute recorded values are dependent of the methodology (i.e., different depth of insertion within a probe and a clinical thermometer), it is well established that rectal temperature displays a diurnal increase whether measured by a rectal probe (Bernard et al. 1998, Racinais et al. 2005b) or clinical thermometer (Atkinson et al. 1993, Racinais et al. 2004, 2005a).
Circadian variations in body temperature are considered to lead many of other circadian variations but can have its own consequences on health, especially during exercise.
During exercise, muscle contraction produces heat leading to a proportional relation within relative work intensity and core temperature (Saltin et al. 1968). For example, during moderate exercise, muscle temperature increases quickly from a basal value close to 35°C, rising above rectal temperature within 3 to 5 min before plateauing after 10 to 20 minutes.
In a neutral environment, core temperature increases when muscle temperature becomes higher than rectal temperature but different thermoregulatory systems attempt to preserve homeostasis. During prolonged exercise the majority of heat dissipation is facilitated by evaporation, while convection, conduction, and radiation play are smaller role.
A part of the heat is also stored in the body leading to an increase in core temperature. However, the storage possibilities are limited and more and more studies state core -and may be cerebral- temperature as the limiting factor of aerobic performance in warm environment (Caputa et al. 1986, Brück and Olschewski 1987, Gonzales-Alonzo et al. 1999). For trained subjects, cessation of activity in warm environment seems to occur at a given critical core temperature whatever the exercise intensity, the heat storage capacity, the acclimatizing state, the initial temperature or the skin temperature (Brück and Olshewski 1987, Gonzales-Alonso et al. 1999).
Accordingly, rats stop exercising in hot environment at the same abdominal and cerebral temperature whatever the modification made to their initial temperature (Fuller et al. 1998, Walters et al. 2000). Furthermore, if the cerebral temperature of a goat is increased close to 42°C by a local warm-up, they reduced their velocity or refuse to move (Caputa et al. 1986).
In human body, from a basal temperature close to 37°C to a limit close to 40°C (Gonzales-Alonso et al. 1999), the heat storages possibilities seem inadequate. From this point of view, the natural circadian variation in core temperature seems to be an important point to consider.
Indeed, an increase in body temperature before a long-duration exercise can decrease performance by altering the heat storage capacity (Nadel 1987, Gonzales-Alonso et al. 1999). Similarly, pre-cooling can increase the heat storage capacity and increase the running time to exhaustion (Lee and Haymes 1995, Gonzales-Alonso et al. 1999). Thus, we can hypothesis that the circadian variation in core temperature will modify the heat storage capacity and thus the ability to exercise.
This hypothesis is valid only when heat dissipation fails to maintained homeostasis and thus the heat storage capacity becomes a limiting factor. This is notably the case in warm and humid environment, when required evaporative cooling exceeds the evaporative capacity of the environment (Shapiro et al. 1998). The diurnal variation in core temperature persisting during physical exercise (Reilly et al. 1984, Reilly and Brooks 1990) and the thermal stress induced by physical exercise in warm (and humid) environment should be higher in the evening, when the initial body temperatures are at its highest (Reilly and Garret 1998).
However, this could auto regulated by the human body. Indeed, it seems that the increase in body temperature during prolonged exercise could be greater in the morning, leading to a reduction in the amplitude of the circadian variation as exercise progress (Aldemir et al. 2000).
Thus, this topic needs further research in warm environments is required to investigate the effect the circadian rhythm in core temperature on thermoregulation capacity during exercise.
This data are coming from: Racinais S. Circadian Rhythms and Physical Activity. In: Progress in Circadian Rhythm Research, Ed: Leglise A.L., Nova Science Publisher, 2008.