A MATHEMATICAL METHOD FOR STUDYING THE ENDOGENOUS COMPONENT OF THE CIRCADIAN TEMPERATURE RHYTHM AND THE EFFECT OF AGING

A mathematical model was developed in order to study the endogenous co mponent of the circadian rhythm in body temperature. The model describ es the fluctuations in body temperature as a function of a cosine-shap ed endogenous rhythm plus an exogenous component which is linearly cor related with the time spent in active wakefulness. The model was evalu ated in 4 young and 4 old rats. In 7 out of 8 rats there was a signifi cant lack of fit when the traditional cosinor method was used, as comp ared with only 1 out of 8 when using our model. In all 8 rats the regr ession was highly significant and also useful as defined by the gamma( m) criterion. The results from the model were in agreement with litera ture regarding constant routine studies in humans. The mean amplitude of the endogenous rhythm was 0.24 degrees C in young rats and 0.19 deg rees C in old rats, whereas the amplitudes of the overt rhythm were 0. 38 and 0.26 degrees C, respectively. The age-related differences in th e amplitude of the overt circadian temperature rhythm could to a large extent be attributed to age-related differences in activity-induced h eat production. Finally, the acrophase of the endogenous rhythm occurr ed 18.7 minutes later than that of the overt rhythm. If applicable to human, the proposed method may form a valuable extension to existing c onstant routine protocols for studying the endogenous circadian rhythm in body temperature.