ABSTRACT
Up to this point in the book, we have talked about circadian rhythms as if organisms exhibited only one circadian rhythm at a time. We have certainly talked about rhythmicity in numerous variables, such as locomotor activity, body temperature, melatonin secretion, and blood pressure-but we have treated each variable as if it were the only variable. It is now time to recognize that circadian rhythmicity is exhibited by many variables simultaneously. This will raise a multitude of new problems. The most obvious problem is perhaps how the multiple rhythms relate to each other-the problem of internal temporal order. Do all rhythms peak at the same time, or do some peak earlier or later than others? Do all rhythms have the same waveform, so that we can legitimately compare their temporal structures? The next obvious problem is that of causation. If one rhythm lags behind another, is it because it is caused by the earlier rhythm? Does the circadian pacemaker generate each and every rhythm individually, or are most rhythms simply derived from a few clock-controlled rhythms? We do not have denite answers to these questions, but research has advanced enough to allow us to at least start to answer them.
