Major depressive disorder (MDD) severely affects patients' psychosocial function and quality of life and causes significant socioeconomic burden (Paykel, Brugha, & Fryers, 2005). The lifetime prevalence of MDD is estimated at 20%, with approximately two thirds of patients experiencing recurrent episodes or a chronic course (Kessler et al., 2010). The vast impact of the disorder on public health is reflected by its recognition as the major cause of disability worldwide (WHO, 2017). Abnormalities in autonomic nervous system activity are considered an important factor in