Decreased Long Term Variations of Heart Rate Variability in Subjects with Higher Self Reporting Stress Scores Desok Kim, Yunhwan Seo, and Lizawati Salahuddin School of Engineering, Information and Communications University Daejeon, Korea kimdesok, gyluarsis, lizawati@icu.ac.kr Abstract— Heart rate variability (HRV) has been well established to measure instantaneous levels of mental stress. Circadian patterns of HRV features have been reported but their relationships to mental stress were not studied explicitly for estimating stress levels. In this study, we investigated long term variations of HRV features to provide a reliable measure of chronic stress levels. Twenty three subjects were divided into high (n=10) and low stress group (n=13) depending their self- reporting stress scores. HRV features were calculated during five different time periods of the day. High stress group showed decreased overall variations of HRV features but similar median values to low stress group. Compared to normal sinus rhythm data during each time period, high stress group showed significantly less % difference of HRV patterns than low stress group. Our data suggested that long term variations of HRV features might be more useful to detect subjects under chronic stress. Keywords-component; mental stress; heart rate variability; Stress Response Inventory; long term rhythm I. INTRODUCTION Chronic stress increases susceptibility to negative health outcomes and involves alteration in behavior, autonomic function and the secretion of hormones such as cortisol [1]. Stress changes the physiological balance of autonomic nervous system (ANS). The ANS is divided into two main divisions: sympathetic and parasympathetic nervous system. Both components operate simultaneously and balance each other dynamically in normal situations. When the stress is given, sympathetic system dominates to increase the heartbeat rates, the perspiring activity of adrenal glands, and breathing rates. To recover from the stress, parasympathetic system takes over to decrease the heartbeat, sweating, and breathing rates. Heart rate variability (HRV) refers to the beat-to-beat alternations in heart rates. HRV is directly associated with the mortality of cardiac patients and diabetic neuropathy [2]. Mental stress decreased HRV features such as high frequency components but increased low frequency components. Thus, HRV analysis has been established as an instantaneous quantitative measure of ANS activity [3]. However, its use in the daily life to monitor chronic stress levels is still limited due to the lack of reliability compared to more conventional methods such as self reporting stress questionnaires and clinical interviews. Stress questionnaires have been used to measure stress levels of individuals in clinical practice and psychological research. Each questionnaire has different characteristics and purposes. In general, questionnaires are used to measure the degree of symptoms, thoughts, feelings, experiences and so on. Stress response inventory (SRI) has been recently devised to score the severity of stress-related symptoms experienced during the previous two weeks that may influence the current status of mental stress. SRI consists of 39 items that focus on the emotional, somatic, cognitive, and behavioral stress responses. SRI scores could be categorized into seven stress factors: tension, aggression, somatization, anger, depression, fatigue, and frustration. Thus, the SRI offers a reasonable method to estimate the levels of chronic stress [4]. The circadian variability of physiological phenomena is well known in the cardiovascular system [5, 6]. Daily variability was studied in sudden cardiac death [7] and myocardial ischemia [8]. Circadian rhythms of ANS under stress were also recently investigated in a few studies: cardiac ANS activity was different while awake and asleep in paramedic workers on the off duty day but not different on the on duty day, suggesting suppressed long term patterns under stress [9]. HRV values were diminished in mice under chronic stress caused by low frequency noise [10] and in swine caged together [11]. In our study, differences of long term HRV patterns between low and high stress groups were investigated to provide a preliminary evidence for the reliable measurement of chronic mental stress levels. Subjects were divided into high and low stress group based on their SRI scores and their HRV features were measured at multiple time points. II. METHODS A. Subjects and Data Acquisition Twenty three students in their 20s and 30s of ages participated in the experiment carried out in Information and Communications University, Daejeon, Korea. Subjects filled This work has been sponsored by Samsung Electronics Co., Ltd., Suwon, Korea.