The role of endogenous circadian rhythmicity in autonomic cardiac reactivity to different stressors was investigated. A constant routine protocol was used with repeated exposure to a dual task and a cold pressor test. The 29 subjects were randomly divided into two groups in order to manipulate prior wakefulness. Group 1 started at 09:00 h immediately after a monitored sleep period, whereas group 2 started 12 h later. Measures of interbeat intervals (IBI), respiratory sinus arrythmia (RSA, a measure of parasympathetic activity), pre-ejection period (PEP, a measure of sympathetic activity), as well as core body temperature (CBT) were recorded continuously. Multilevel regression analyses (across-subjects) revealed significant (mainly 24 h) sinusoidal circadian variation in the response to both stressors for IBI and RSA, but not for PEP. Individual 24 + 12 h cosine fits demonstrated a relatively large interindividual variation of the phases of the IBI and RSA rhythms, as compared to that of the CBT rhythm. Sinusoidal by group interactions were found for IBI and PEP, but not for RSA. These findings were interpreted as an indication for endogenous circadian and exogenous parasympathetic (vagal) modulation of cardiac reactivity, while sympathetic reactivity is relatively unaffected by the endogenous circadian drive and mainly influenced by exogenous factors.

We examined the effects of serum insulin levels on vagal control over the heart and tested the hypothesis that higher fasting insulin levels are associated with lower vagal control. We also examined whether experimentally induced increases in insulin by beta cell secretagogues, including glucagon-like peptide-1 (GLP-1), will decrease vagal control.

Previous research has demonstrated that hearing impairment is associated with heightened subjective experiences of listening effort, fatigue, and stress, impacting daily functioning. This study aimed to evaluate whether hearing impairment alters physiological stress systems and whether different aspects of hearing impairment could vary in predicting dysregulation in these systems. Hallmark measures of parasympathetic and sympathetic nervous system activity were derived from electrocardiography, impedance cardiography, and electrodermal activity recordings taken from 133 individuals, aged 37 to 73, over two 24-hr periods, including sleep. Using ecological momentary assessment (EMA), participants reported mood, listening effort, and fatigue seven times daily. Hearing impairment was quantified through pure tone thresholds, speech perception in noise testing, and the Amsterdam Inventory for Auditory Disability questionnaire (Amsterdam Inventory). Using mixed models, we compared average daytime and sleep values of physiological measures across the 2 days, and their daytime-to-sleep contrast, based on each hearing impairment assessment. Results indicated that all three hearing impairment assessments were strong predictors of EMA outcomes of listening effort and fatigue. Contrary to expectations, hearing impairment did not have a significant impact on parasympathetic activity in daily life or on skin sympathetic activity. However, individuals with higher impairment exhibited a larger change in a cardiac sympathetic measure, the pre-ejection period, during wakefulness compared to sleep. Overall, hearing impairment had a small impact on autonomic nervous system functioning in daily life, but the effects were potentially attenuated by reduced exposure to listening demand in those with hearing impairment.