ABSTRACT
In Section II of this book, we looked at the phenomenology of biological rhythms. We examined ultradian and infradian rhythms in Chapter 4 and then focused on daily and circadian rhythms in Chapter 5. We saw that virtually every biological function ever measured was found to exhibit daily rhythmicity. We also saw that, whereas all daily rhythms are modulated by the alternation of day and night, many rhythms are generated endogenously. We saw that, under constant environmental conditions, the rhythms freerun with periods slightly different from 24.0 hours. For instance, we saw that the rhythm of locomotor activity can freerun in numerous species of invertebrates,1-28 reptiles,29-37 shes,38-44 birds,45-59 and mammals,60-117 including humans.118-129
Because geophysical cycles on Earth are expected to have 24.0-hour periods, the existence of circadian rhythms with periods different from 24.0 hours provides strong evidence in support of the hypothesis of endogenous rhythmicity. For the indulgence of skeptical readers, however, I should add that circadian rhythms have been recorded in humans kept in underground bunkers and caves,126,130 as well as in space.131-133 Also, free-running rhythms were recorded in people living in Arctic and Antarctic eld camps, where Earth’s inuence is just as strong, but there is continuous sunlight throughout the summer and continuous darkness throughout the winter.121,122 A little later, in Section 6.2, we will see that the period of circadian rhythms can be modied by genetic manipulation. I hope, therefore, that all readers accept the fact that circadian rhythms are endogenously generated.
