Circadian Rhythms

Terry L. Page

Vanderbilt University

Circadian rhythms are daily oscillations in physiology, metabolism, or behavior that persist (or free run) in organisms that have been isolated from periodic fluctuations in the environment. These rhythms are under the control of innate regulatory systems that are based on internal oscillators (or pacemakers) whose periods approximate those of the naturally recurring 24-h environmental cycles. The oscillators are subject to control by a limited number of these environmental cycles that synchronize or entrain the period to exactly 24 h and establish specific phase relationships between the rhythms and the external world (Fig. 1). Light cycles are virtually universally effective in the entrainment of

FIGURE 1 Event recording of the wheel-running activity of a cockroach, L. maderae. Data for successive days are placed one below the other in chronological order. The bar at the top of the record indicates the light cycle to which the animal was exposed during the first 14 days of the recording. Then animal was then placed in constant darkness (DD) and its endogenously generated, free-running circadian rhythm was expressed for the remainder of the record with a period of about 23.5 h.

FIGURE 1 Event recording of the wheel-running activity of a cockroach, L. maderae. Data for successive days are placed one below the other in chronological order. The bar at the top of the record indicates the light cycle to which the animal was exposed during the first 14 days of the recording. Then animal was then placed in constant darkness (DD) and its endogenously generated, free-running circadian rhythm was expressed for the remainder of the record with a period of about 23.5 h.

circadian rhythms, and in insects, daily cycles of temperature are also effective.

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