ABSTRACT
Part II of this book looked at the phenomenology of biological rhythms. Ultradian and infradian rhythms were examined in Chapter 4, and daily and circadian rhythms were discussed in Chapter 5. Part II showed that nearly every biological function ever measured exhibits daily rhythmicity. While all daily rhythms are modulated by the alternation of day and night, many rhythms are generated endogenously. Under constant environmental conditions, the rhythms freerun with periods slightly different from 24.0 hours. For example, the rhythm of locomotor activity can freerun in numerous species of invertebrates,1-13 reptiles,14-21 fishes,22-25 birds,26-39 and mammals,40-91 including humans.92-102
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. In addition, circadian rhythms have been recorded in humans kept in underground bunkers and caves,100,103 as well as in space.104-106 Also, free-running rhythms were recorded in people living in Arctic and Antarctic field camps, where Earth’s influence is just as strong but continuous sunlight exists throughout the summer and continuous darkness occurs throughout the winter.95,96
If circadian rhythms are endogenously generated, a clock that generates them must exist. Everyone knows what a clock is, but no one expects to find a clock such as that shown in Figure 6.1 inside an organism. Let me clarify what circadian physiologists mean by the word clock. Three basic elements of time measurement include the ability to undergo a constant change of state over time, the ability to display absolute time, and the ability to generate a self-sustaining oscillation. The three circles in Figure 6.2 represent these elements. The lower circle represents the ability to undergo a constant change of state
over time. Many entities in the world do not demonstrate this capability. For example, water evaporates at 100°C today, it did so yesterday, and it will do so tomorrow. Other entities do change over time, and they do so at a constant rate (or at least in a predictable manner). This property is sufficient to define a timer. Two examples of timers include an hourglass and a mechanical count-down timer.
