ABSTRACT
This chapter focuses on some topics relate to the concepts of freerunning circadian rhythms. They are: development, aging, accuracy, the clock and hands problem, two views, and inherited clocks. The amplitude of many rhythms decreases as old age approaches. Usually this decrease is a reduction of the maxima for the rhythm, for example, hamster pineal melatonin is nearly 1000 units at night in 2-month old hamsters, but less than 200 units in 18-month old hamsters. Enright considered the precision of the circadian biological clock and reported that three canaries, for example, had individual cycle-to-cycle accuracies with standard deviations of 3, 4, and 7 minutes. He described this as an error of as much as one part in 200 or as little as one part in 500. Genetic studies support the idea that circadian rhythms are inherited. Although there are some aftereffects of timed pretreatments, new frequencies are not learned.
