Abstract
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°C to 38·45 ± 0·25°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<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.