Final answer:
Michel Siffre learned that in isolation without external cues, the human circadian rhythm extends beyond the standard 24 hours, suggesting a natural free-running cycle.
Step-by-step explanation:
Michel Siffre, a chronobiologist, conducted an experiment in 1962 where he spent two months in total isolation in a cave without access to sunlight or clocks to study the circadian rhythm. His research revealed that in the absence of external time cues, the human biological clock tends to follow a cycle slightly longer than the traditional 24 hours, indicating an innate free-running rhythm. Siffre found that without external time cues, such as daylight, his internal body clock adjusted to a longer cycle. This resulted in his perception of time slowing down, with days feeling like weeks. This experiment demonstrated how our circadian rhythms can be influenced by our environment and isolation.
Siffre's experiment underscored the importance of external cues, such as daylight, in regulating the human sleep-wake cycle. Without these cues, Siffre experienced altered perceptions of time and intervals between sleep. The experiment became a landmark in understanding the internal time-keeping mechanisms and the concept of entrainment to environmental cues. This research helped pave the way for further studies in chronobiology and also has implications for those experiencing isolation, such as astronauts and individuals in polar environments.