FIGURE 5.26 Circadian rhythm of temperature selection.

The graph shows the body temperature rhythm of a small iguana lizard (Iguana iguana) maintained under constant light in a room at 22°C. The animal could change its body temperature by moving on and off a "hot rock" (i.e., an electric heater). The records show a clear oscillation in temperature with a period close to 24 hours. (Source: Data collected by R. Refinetti at M. Menaker's laboratory at the University of Virginia in 1991.)

selection resulted in a circadian rhythm of body temperature (as measured by a temperature-sensitive radio transmitter implanted in the abdomen).

Time perception is another phenomenon that has received considerable attention. Humans often use watches and clocks to measure the passage of time (Figure 5.27). These instruments allow individuals to time events without constantly thinking about them. However, humans and other animals are quite capable of estimating the passage of time in the range of seconds, minutes, or hours.383 This capability seems to depend on circadian rhythms. The dependence occurs at two levels. First, the ability to estimate short durations of time exhibits daily and circadian rhythmicity. An example is shown in Figure 5.28. Fourteen young men were each kept in bed from 7 a.m. to 11 p.m. Each was asked to estimate the duration of 10 seconds at four different times of the day. They overestimated the duration at all times, but the overestimation was greatest in the morning.384 Greater overestimation in early morning also was seen in other studies in which subjects were asked to estimate the duration of 10 seconds385 or of 1 hour.386 However, in one study the highest estimate occurred early in the night instead,249 and in another the peak times were not consistent across subjects, even though all subjects exhibited daily rhythmicity in their estimates.387 In yet another study, the researchers investigated the "personal tempo" of subjects asked to tap their fingers at their preferred rate. The average rate of 2.3 taps per second exhibited a daily variation of about 30% with a peak at 7 p.m.388 It seems well established that time perception exhibits daily rhythmicity, but further studies are necessary to determine the precise waveform of the rhythm.

Time perception also depends on circadian rhythms in subjects kept in temporal isolation. Estimates of the duration of an hour were found to be longer when the

FIGURE 5.27 What time is it? One reason why humans routinely use clocks to measure the passage of time may be that our perception of time intervals varies with the time of day. (Source: © ArtToday, Tucson, AZ.)

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