Heart rate variability in chronic and acute stress : with special reference to nocturnal sleep and acute challenges after awakening

Hynynen, Esa Heart rate variability in chronic and acute stress with special reference to nocturnal sleep and acute challenges after awakening Jyvaskyla: University of Jyvaskyla, 2011, 74 p. (Studies in Sport, Physical Education and Health, ISSN 0356-1070; 163) ISBN 978-951-39-4207-6 (PDF), 978-951-39-4194-9 (nid.) Finnish summary Diss. The present study was designed to investigate the association between chronic and acute forms of physical and psychological stress and cardiac autonomic modulation in real-life situations. The results showed that chronic physical stress (overtraining) did not affect nocturnal autonomic modulation measured either by stress hormone secretion or heart rate (HR) and/or heart rate variability (HRV) indices. HRV measured immediately after awakening in an orthostatic test was diminished in the presence of chronic physical stress. Furthermore, overtrained athletes showed lower cognitive performance and HRV responses to a cognitive task and relaxation after the task than control athletes. In contrast to chronic physical stress, acute physical stress produced through endurance exercises was found to be related to a decrease in nocturnal HRV. In addition, nocturnal HRV responses to the endurance exercises had a dose-response relationship. Chronic psychological stress was found to be associated with lower HRV after awakening in an orthostatic test, but no difference was found in nocturnal HR, HRV and stress hormone secretions between low and high stress groups. Acute psychological stress was related to high HR response during a parachute jump and further, HR was found to be higher in novices than in experienced jumpers both in absolute units and in relation to age-predicted maximum HR. Despite of the high sympathetic activation during the acute stress, no association was found to any HR or HRV indices during night sleep and orthostatic tests after awakening. Slightly elevated adrenaline secretion was found during the night preceding the jump, but it may be explained by seasonal variation, and was therefore not related to the psychological stress. These results indicate that both chronic physical and chronic psychological stress can diminish HRV, especially during the awakening period in the morning. This phenomenon may be used to estimate the stress-induced withdrawal of parasympathetic autonomic modulation which may have negative effects on human health. Furthermore, nocturnal HRV may be used to estimate changes in autonomic modulation during the recovery phase from acute endurance exercises. These methods can be utilized in individual follow-ups of stress management or athletic training programs.