Spontaneous short-term fluctuations in the daily pattern of heart rate, body temperature and locomotor activity in the laboratory rat

Heart rate (HR), body temperature (BT) and locomotor activity (LA) were measured continuously over 5 days in freely moving rats. In addition to the well-known circadian rhythms, all variables exhibited considerable fluctuations in amplitude mainly during the dark, but also in the light periods. The values of HR varied from 286 +/- 12 to 470 +/- 26 b.p.m. and BT from 36.15 +/- 0.15 degrees C to 38.45 +/- 0.25 degrees C. The large variability of HR, BT and LA within a single day was due more to large short-term fluctuations within periods of about 3-5 hours duration, than to differences between the light and the dark period. Good consistency of daily patterns and similarity of the 3 variables was found within the animals. Usually there were 3 or 4 regular peaks during the dark and often another peak 3-4 hours after the onset of light. Correlation coefficients, calculated on the basis of 5-min mean values, were highly significant (P less than 0.001) for LA vs HR (0.61-0.73), LA vs BT (0.40-0.53), and HR vs BT (0.61-0.68). Between-hour correlations were higher than these common correlations of 5-min values. HR vs BT (0.76-0.83) and LA vs BT (0.63-0.79) correlated as well as LA vs HR (0.72-0.83). The short-term fluctuations (within-hours) gave lower correlation coefficients for LA vs BT (0.23-0.32) and HR vs BT (0.29-0.41) than LA vs HR (0.40-0.70). This seems to result from a physiological delay of BT relative to HR and LA.