Circadian variation in base rate measures of cardiac autonomic activity

To investigate the role of the circadian pacemaker in autonomic modulation of base rate cardiac activity, 29 healthy subjects participated in a constant routine protocol. They were randomly divided into two groups in order to manipulate prior wakefulness. Group 1 started at 0900 hours immediately after a monitored sleep period, while group 2 started 12 h later. Measures of interbeat intervals (IBIs), respiratory sinus arrythmia (RSA, an estimate of parasympathetic activity), pre-ejection period (PEP, an estimate of sympathetic activity), and core body temperature (CBT) were recorded continuously. Multilevel regression analyses (across-subjects) revealed significant 24- and/or 12-h sinusoidal circadian variation for CBT, IBI, and RSA, but not for PEP. Subject-specific 24+12 h sinusoidal fits demonstrated a convergence of phase distribution for IBI and RSA of group 1 similar to CBT, while PEP showed a relatively large (i.e. random) distribution of phase. In group 2, all cardiac measures showed large distributions of phase. Unexpected results in the cardiac measures were found in group 2, probably caused by group differences in prior activation. Also, effects of sleep deprivation were observed for IBI and RSA in group 2. Consequently, all cardiac measures revealed significant sinusoidal × group interactions, a result not shown in CBT. These findings were interpreted as an indication for circadian endogenous parasympathetic modulation of cardiac activity that is mainly confounded by prior wakefulness that extends 24 h, while the sympathetic modulation is relatively uncoupled from the endogenous circadian drive and mainly influenced by prior activation.