Scientists also soon discovered the importance of this cyclicity in most of the body’s major systems. Whatever physiological variables researchers measured—such as cell metabolism, body temperature, or the secretion of various hormones—each seemed to fluctuate in a cycle with its own specific peaks and troughs.
Many experiments have been conducted to uncover the subtle connections among these various rhythms. Some of the greatest insights have been provided by temporal-isolation experiments, in which subjects are completely isolated from the usual cues of alternating day and night, such as daylight and traffic noises. Numerous researchers have conducted such experiments, to examine questions such as whether, under such conditions, people continue to fall asleep at their usual time, or whether their activity cycle instead begins to run too fast or too slow.
The Cave study: Michel Siffre (1972)
Aim: This study investigated what would happen to people’s circadian rhythms if they were cut off from all zeitgebers (signals from outside the body that tell us about the time of day – such as light and dark, clocks), and had to rely on their endogenous pacemaker (internal body clock) to tell them when to eat and sleep. Would we still stick to a natural 24-hour rhythm?
Method: Michel Siffre, a French cave explorer, spent over six months living in a cave in Texas, deep under the ground, with no light, or anything else to tell him what time of day it was. His biological clock was allowed to ‘free-run’, that is, he just followed his body’s inclinations, eating and sleeping whenever he chose, with no fixed timetable. He was wired up so that some of his body functions could be recorded; he had a telephone link to the outside world, and was monitored by video camera.
Results: Siffre had a fairly erratic sleep-wake pattern at first, but it settled down to a pattern that averaged just over 25 hours, instead of 24 hours.
Conclusion: We do have an internal mechanism that regulates our sleep/wake cycle, but it shifts to a length of approximately 25 hours if we do not have external zeitgebers to reset it.
Evaluation: This is a one-participant study, so may not be generalisable to all humans. Also Siffre’s living conditions were unusual in other ways than simply lacking time signals, and other factors such as loneliness could have affected his behaviour. Similar studies have been done with rats, isolating them from daylight (Groblewski), and found a similar increase in the sleep-wake cycle, which supports the findings from the Siffre study. A strength of the study is that it lasted a long time, allowing Siffre’s rhythms to settle down into a natural pattern.