The Human Clock

Since the early experiments, chronobiology--the study of biological rhythms--has become established as an interdisciplinary field within biology. Most chronobiologists study circadian rhythms, endogenous cycles of behavior or biological activity with a period of about 24 hours. In the example (figure 1),the human sleep-wake cycle has a period of 1 day, or goes through 1 complete cycle in a day. Circadian rhythms, like the sleep-wake cycles discussed later, are generated by an internal clock that is synchronized to light-dark cycles in the environment and other daily cues.

Figure 1

	Circadian rhythms are frequently plotted on an actogram(figure 2). An entraining agent (for example, exposure to bright light) can cause a phase shift (dotted line in the figure 3) whereby the activity is started earlier or later in the day.

Figure 2	Figure 3

Like a watch, the circadian clock must be synchronized to local time. For example, animals kept in total darkness will show a free running rhythm that is independent of the local time. A circadian clock is most useful, however, when it is set to local time; the animal must be in sync with its prey, pollinators, and other members of its social group in order to survive. In mammals, the light-dark cycle is a major synchronizer or entraining agent for circadian rhythms[2].