Differences in the 24-h autonomic nervous activity of outpatients with major depressive disorder with or without hypersomnia: An exploratory observational study

Hypersomnia, defined as a total sleep time of over 10 h within a 24-h period, is common in patients with major depressive disorder (MDD). The circadian rhythm of patients with hypersomnia and depression is disturbed; however, differences from patients without hypersomnia remain unclear. We aimed to clarify differences over 24 h in the autonomic nervous system (ANS) activity of patient groups with MDD with or without hypersomnia. This study included outpatients with MDD who were categorized into either a hypersomnia or non-hypersomnia group based on the Diagnostic and Statistical Manual of Mental Disorders-5 criteria. Heart rate variability data were collected over 24 h using electrocardiograms. Differences in ANS activity between groups across bedtime and non-bedtime periods were analyzed using two-way ANOVA. General linear models were used to compare hourly 24-h data, with heart rate; natural logarithm (ln) of the low frequency domain (LF) as sympathetic and parasympathetic nerve activity; ln of the high frequency domain (HF) as parasympathetic nerve activity; ln LF/HF as objective variables indicating sympathetic domination; hypersomnia status every hour during the 24-h period as explanatory variables; and age and body mass index as variable effects. Twenty-eight participants were enrolled; the data of 25 were available for the final analysis. No interaction between hypersomnia status and period was observed for ANS indices. ln LF differed significantly by hypersomnia status but not by period (bedtime vs. non-bedtime); the hypersomnia group showed higher ln LF across both periods. In the evening, ln HF was significantly higher in the hypersomnia group than in the non-hypersomnia group, whereas ln LF/HF was significantly higher in the early morning in the hypersomnia group. No interaction between hypersomnia status and period (bedtime and non-bedtime) was observed for autonomic nervous activity indices. However, in the hypersomnia group, hourly analyses showed significantly higher ln HF from 17:00 to 22:00 h and higher ln LF/HF before waking (3:00–4:00 h) compared with the non-hypersomnia group. ANS activity differed depending on presence or absence of hypersomnia, a risk factor for relapse and bipolar disorder.