Self-monitoring by wearable biosensors is thought to be a form of biofeedback during daily activity. Analysis of heart rate variability (HRV) from ambulatory electrocardiogram or pulse wave is widely used for this purpose, i.e., the self-control of autonomic functions under various conditions such as sleep, relaxation, exercise, and stress. However, there is a question whether the principle of autonomic assessment by HRV that was developed on short-term HRV under controlled conditions can be directly applied to HRV under uncontrolled daylily activities. Many recent studies have reported results that do not support this. More attention needs to be paid to the application, analysis, and interpretation of HRV in daily life. Conversely, the long-term monitoring of HRV during daily life also has advantages over short-term HRV. That is the evaluation of the reflex autonomic functions using the heart rate responses to accidentally occurring biological phenomena during daily life, such as ventricular ectopic beats and sleep apnea episode. This paper describes the limitations and usefulness of long-term monitoring of biological functions, focusing on heart rate and HRV.
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