The observed pattern of diminished linear HRV and increased nonlinear HRV is similar to that seen in subjects undergoing heavy exercise or in patients with ischemic heart disease at high risk for ventricular fibrillation.

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It is accepted that several biological (genetic, physiological), psychological (personality, mood), social (family, work) and ecological (living environment) factors, interact to allow the preservation of quality of life and health. The autonomic nervous system (ANS) is structural and rhythmically interfaced between forebrain, internal and external environments, to regulate energy, matter and information exchanges, thus expressing the bio-psycho-social nature of the individual. During last year's strong evidence has been produced that demonstrate the influence of the ANS into beat to beat fluctuations of cardiac heart period. Stephen Hales was the first in describing the association between respiratory cycle and heart rate. However, it was not until the second half of the XX century that subtle relations between ANS activity and heart rate variability (HRV) were systematically explored in several physiological conditions, including physical activity, sleep and stress. In addition, autonomic imbalance may configure a final common pathway to increased morbidity and mortality from a host of physical, such as metabolic disorders and cardiovascular disease, and psychological conditions, like anxiety or depression. This chapter will focus on the physiological mechanisms underlying HRV, the methods for assessing it and the information that has provided about ANS activity in certain physiological and pathological situations.