circadian

circadian clock An internal time-keeping system in all organisms. Changes in the external environment, particularly in the light–dark cycle, train this biologic clock. When environmental conditions are constant, rhythms driven by the circadian clock follow a nearly perfect 24-hour pattern. The human circadian clock regulates many daily activities, such as sleep and waking. When a person doesn’t follow these natural rhythms, or when the external environment strays from its usual rhythm (as occurs in the long nights and short days of deep winter), the circadian clock must readjust. Rapid environmental changes and problems with circadian clock adjustment are among the causes of jet lag, problems that affect shift workers, some types of sleep disorders, and bipolar disorders, particularly seasonal affective disorder. Certain genes serve to set and control the circadian clock. See also bipolar disorder; jet lag; seasonal affective disorder; sleep disorder.