In free-running cycles, periods can vary among species, among individuals of the same species, and even among different systems in the same individual. These periods can also be affected by disease, drugs, or by social interaction among free- running individuals. Many, if not most, of the biological cycles we observe in nature depend on chemical cycles that arise within organisms. We will call these cycles that arise from within "biological rhythms". Under normal conditions, the environmental changes influence these rhythms, continually adjusting their peak times to keep them in step. Like the beat of a parade drum, the environmental cycles serve to keep "in step" cycles that are capable of "marching" by themselves. We say that internal rhythms are usually synchronized by environmental influences.
The environment, however, has only limited influence over internal biological rhythms. Although circadian cycles in some plants can be squeezed by artificial lighting schedules into "days" as short as four hours, most processes coordinated by internal circadian cycles cannot be forced into "days" shorter than about 18 hours or longer than about 36 hours. If an organism is given a lighting schedule with a period that is drastically different from its internal rhythm, the organism breaks free from the synchronizing control and goes back to its usual free-running period, usually not more than three hours different from 24 hours. Hamsters, for example, have a narrow range — their days cannot be compressed or stretched beyond 21-26 hours. In summary, many biological rhythms arise from within organisms, although they are typically influenced by the environment in period, amplitude, and peak time. Where is the internal source for timing information within an organism? How are internal rhythms coordinated?
Halberg Chronobiology Center > Outreach & Education > Cycles of Nature > Cycles of Nature: The Role of Environmental Influences >