Diurnal patterns of heart rate variability and associations with markers of mental health in South Africans living in a low-income setting

Introduction: Heart rate variability (HRV), indicates the relative contributions from the sympathetic and parasympathetic branches of the autonomic nervous system and exhibits diurnal variation in healthy individuals, increasing during sleep and decreasing during wake. This is attributable to an expected increase in parasympathetic tone (“rest-and-digest” response) during sleep and an increase in sympathetic tone (“fight-or-flight” response) during wakefulness. People residing in low socioeconomic status (SES) environments with high rates of exposure to stressful events, may be in a constant state of hyperarousal, characterised by impaired or dampened diurnal HRV patterns with reduced HRV during sleep. Damped diurnal variation has previously been linked to both poorer sleep and more severe depressive symptoms. The aims of this study were to i) characterize the diurnal variation in HRV over a 24-hour period in South Africans living in a low SES environment and ii) investigate associations between parameters of diurnal variation in HRV with symptoms of depression, anxiety, and post-traumatic stress disorder, which are known to occur widely in this setting.

Materials and methods: Thirty-four African-origin South Africans (25-45y, 75% women) were recruited as part of a larger, longitudinal parent study: the “Modelling the Epidemiologic Transition Study (METS)-Microbiome” study. Demographic and mental health data (Fear of Sleep Inventory, Primary Care for PTSD Screen, Becks Depression Inventory and Becks Anxiety Inventory) were collected while 24h-ambulatory electrocardiography and actigraphy were used to measure HRV and nocturnal sleep, respectively. Actigraphy was used to set the sleep period to compare sleep and wake HRV. HRV data was manually inspected for artifact then 5-minute epochs were averaged in 15-minute intervals for each hour. Sine curves were fitted to the hourly HRV variables to characterise amplitude, mesor, time of peak and time of nadir. Spearman’s correlations were used to investigate associations between HRV measures and mental health outcomes.

Results: Actigraphy-derived mean (±standard deviation) sleep duration and sleep time were 9.1±1.0h and 6.9±1.2h respectively. Three diurnal patterns of HRV emerged: rhythmic, dampened and bimodal. Decreased HF amplitude (rho:-0.562, p=0.015) and RMSSD amplitude (rho:-0.551, p=0.018) were associated with more severe symptoms of anxiety. Higher levels of fear of sleep (rho:-0.535, p=0.022), depression (rho:-0.556, p=0.017) and anxiety (rho:-0.560, p=0.016) were all associated with time at which peak in very low frequency (VLF) occurred.

Conclusions: We show dampened diurnal variation in multiple HRV variables in a vulnerable group of South African adults, associated with more severe mood- and anxiety-related symptoms. Specifically, the delayed timing of the VLF peak indicates intrusion of sympathetic activity or delayed increase of parasympathetic activity during the sleep period, which may have implications for sleep quality. This may partly explain the short sleep observed despite sufficient opportunity. Hyperarousal or hypervigilance may in part contribute to impaired HRV during both wake and sleep in these individuals, perpetuating the poor sleep-poor mental health cycle.