The aim of this study is to investigate the correlation between saliva cortisol concentration and balance of autonomic nervous system (sympathovagal balance) before sleep in normal male subjects. The subjects (N=27) intermittently wore an ambulatory wearable device that records heart rate variability (RR-interval) for three days during daily life. Their saliva was sampled at each night before sleep. The cortisol concentration of each sample was quantified. The sympathovagal balance was evaluated by calculating the normalized high-frequency power (HFnu) of RR-interval variability that was measured from A+5 to A minutes (A=0, 5,..., 55) before the saliva sampling time. The correlation coefficients between high values of saliva cortisol concentration and the corresponding natural logarithm of HFnu were from -0.40 to -0.49 (p=0.006-0.034) when the values of A were 10, 15, 20, and 25. This result implies that hypothalamic-pituitary-adrenal (HPA) axis activity before sleep is significantly correlated with dominance of sympathetic nervous system activity with time lag of 10-30 minutes during relatively high stress condition.
