The Circadian Rhythm of Core Temperature: Origin and some Implications for Exercise Performance

Abstract

This review first examines reliable and convenient ways of measuring core temperature for studying the circadian rhythm, concluding that measurements of rectal and gut temperature fulfil these requirements, but that insulated axilla temperature does not. The origin of the circadian rhythm of core temperature is mainly due to circadian changes in the rate of loss of heat through the extremities, mediated by vasodilatation of the cutaneous vasculature. Difficulties arise when the rhythm of core temperature is used as a marker of the body clock, since it is also affected by the sleep-wake cycle. This masking effect can be overcome directly by constant routines and indirectly by "purification" methods, several of which are described. Evidence supports the value of purification methods to act as a substitute when constant routines cannot be performed. Since many of the mechanisms that rise to the circadian rhythm of core temperature are the same as those that occur during thermoregulation in exercise, there is an interaction between the two. This interaction is manifest in the initial response to spontaneous activity and to mild exercise, body temperature rising more quickly and thermoregulatory reflexes being recruited less quickly around the trough and rising phase of the resting temperature rhythm, in comparison with the peak and falling phase. There are also implications for athletes, who need to exercise maximally and with minimal risk of muscle injury or heat exhaustion in a variety of ambient temperatures and at different times of the day. Understanding the circadian rhythm of core temperature may reduce potential hazards due to the time of day when exercise is performed.

The effects of single versus twice daily short term heat acclimation on heat strain and 3000m running performance in hot, humid conditions

“…Nonetheless, when comparing our predefined analytical limits (>0.2ºC) and effect size between session 1 and 4, meaningful reductions of 0.4% were present in the SDHA group similar to that reported elsewhere (Sunderland et al, 2008;Petersen et al, 2010). Similar results were present in T repeak reductions of 0.7% within the SDHA but not TDHA group, as comparison between sessions 1 to 4 are contributed by circadian fluctuations in T re (Waterhouse et al, 2005). No change in T re occurred in the CON group.…”

Maternal temperature during labour

“…It is well known that temperature follows a circadian pattern with higher values during daytime than during night-time. 23 Acker et al observed a circadian pattern of temperature at onset of labour, although at a slightly lower level, probably because they measured temperature orally. 24 In our study, admission temperature and mean temperature followed a circadian pattern.…”

“…[36][37][38] Although rectal measurement has some inconvenience, we decided on this method as it is generally considered to be gold standard for clinical temperature measurement. 23,35,37 Conclusions At the beginning of labour, temperature was 37.1°C (2SD 0.7). Temperature increased slowly during labour and was 37.4°C (2SD 1.2) after 22 hours.…”

Chronobiological characterization of women with primary vasospastic syndrome: body heat loss capacity in relation to sleep initiation and phase of entrainment

“…The suprachiasmatic nuclei drives circadian rhythms in physiological processes, which are synchronized to the outside world mainly by the solar light-dark cycle. Core body temperature (CBT) has been investigated as a robust and convenient circadian marker rhythm in humans (4,64,67). However, the circadian rhythm of CBT is more than a marker (17,63); thermoregulatory changes are intimately coupled to sleepiness and sleep induction (see below).…”