Heart Rate Variability
Variation may be viewed as a fundamental feature of nature and especially so of living organisms. Some features of variation are easily appreciated, for example in the individual diversity of human characteristics or in the change of the seasons. All aspects of human physiology are underpinned by interlocking cycles each with intrinsic frequencies from the beating of our heart, the subject of this monograph, to the timing of daily sleep. With respect to the latter, the human wake/sleep cycle, also known as the circadian cycle, is nearly 24 hours in length and, because it is determined genetically, is quite stable over one’s lifetime. Its activity is kept stable relative to the solar day primarily by exposure of sunlight on the retinae. The timing of many biorhythms such as hormonal secretion, body temperature, gastrointestinal activity, blood pressure, blood sugar metabolism, urine production, mental alertness and EEG activity are related in some way to the circadian cycle. Also connected directly to the circadian cycle are heart and respiratory rates. Our heart beat normally varies, within a narrow range, beat to beat. This variability is driven by the brain through the autonomic nervous system (ANS) and can be shaped by our breathing patterns. Unlike many other physiological activities, control of breathing is something that most people can learn to do. Doing so is a simple and cost-free way of improving health.