“…Heart rate variability (HRV) is an effective indicator of the activities of the autonomic nervous system (i.e., the balance between sympathetic and vagal activity) and is widely used to assess the autonomic response to various internal and external factors ( Acharya et al, 2006 ; von Borell et al, 2007 ; Billman, 2011 ). Indeed, the responses to a variety of physiological, psychological, and clinical factors such as exercise effects ( Pradhapan et al, 2014 ; Valenzano et al, 2016 ), anxiety ( Chalmers et al, 2014 ; Cotoia et al, 2018 ), and health conditions (e.g., cardiac diseases, Watanabe et al, 2017 ; Ciliberti et al, 2018 ; Sessa et al, 2018 ; viral infection, Carter et al, 2014 ; and obesity, Mehta, 2015 ; Messina et al, 2017 ; Triggiani et al, 2017 ) have been intensively investigated using HRV analysis in recent human studies. However, HRV is simultaneously affected by several interdependent physiological and environmental factors ( Fatisson et al, 2016 ) because the sinus node acts as the final summing element of stimuli from the sympathetic and vagal nerves, and their relationship is reflected in the actual heart inter-beat intervals ( Voss et al, 2009 ).…”