Human Biological Rhythms: The Search for Ultradians in Movement Activity Behaviour

Abstract

Human biological rhythms are the expressions of orderliness governing the proper functioning of the organism throughout its existence. The most familiar of these rhythms is the circadian (cycling about once a day) by virtue of its immediacy in generating the pattern of daily life. It is centred around a period of approximately 24 h, give or take an hour or two in either direction. It is the easiest to track in that under normal circumstances humans dutifully follow the sleep-activity cycling within each 24-h day. When this circadian rhythm is disrupted by whatever reason (i.e. continuous wakefulness (or sleep), transmeridian flight across several time zones, illness, etc.), the individual feels “out of sorts” or desynchronized from his or her ordinarily constant patterns. This is because many of the major physiological measures in humans also follow the same circadian rhythmicity as the sleep-activity cycle. These include body temperature, heart rate, movement activity, brain activity, cognitive performance, levels of certain hormones, enzymes, and blood constituents. When the circadian rhythms of these major regulators of normal functioning for humans are perturbed, then a system accustomed to running “like clockwork” may be on the verge of disorder. But the human is an eminently adaptable entity and unless the perturbation is extraordinarily severe and prolonged, circadian rhythms are sufficiently labile to allow adjustment to the new circumstances over time.