Attenuated parasympathetic modulation during “Wake to Sleep” and “Sleep to Wake” in high-neuroticism young adults: a heart rate variability study

Neuroticism, characterized by emotional instability and heightened stress sensitivity, has been associated with sleep disturbances and altered autonomic nervous system regulation. However, its impact on dynamic parasympathetic modulation during sleep–wake transitions defined here as periods immediately surrounding sleep onset and morning waking remains unclear. Healthy young adults (62 men and 49 women) were classified into a control or neuroticism group based on Kudo’s two-dimensional discriminative chart for the Cornell Medical Index. Participants in Groups I–II were assigned to the control group (n = 99), and those in Groups III–IV to the neuroticism group (n = 12). Heart rate variability (HRV) was measured using a portable electrocardiographic, and parasympathetic activity was indexed by normalized high-frequency component (HFnu). Two transition-based indices were calculated: ΔSleepOnset, (HFnu during the 90 min after minus before sleep onset) and ΔWake (HFnu during the 90 min after minus before waking). Both ΔSleepOnset and ΔWake were significantly smaller in the neuroticism group (ΔSleepOnset: p = 0.040, d = 0.20; ΔWake: p = 0.041, d = 0.20), whereas average HFnu during the entire sleep period did not differ significantly between groups (p = 0.28, d = 0.10). The same pattern was observed when analysing Group III alone, although statistical power was limited by the small size of Group IV. Despite comparable overall parasympathetic activity during sleep, neuroticism group exhibits blunted parasympathetic modulation at sleep–wake transitions, reflecting reduced autonomic flexibility. Transition-based HRV indices such as ΔSleepOnset and ΔWake may serve as sensitive physiological markers of sleep vulnerability associated with neurotic traits.