Circadian rhythms are ones which last roughly 24 hours. They are affected by external cues, known as Exogenous Zeitgebers; these include light, food, noise and social interaction. They are also controlled by Endogenous Pacemakers, which are internal "clocks" which affect these rhythms. Two examples of circadian rhythms include both the sleep/wake cycle.
The sleep wake cycle is governed by both exogenous zeitgebers (noise, light) and endogenous pacemakers (internal factors). The internal clock is free running and works without any external cues. A study by SIFFREE has shown that circadian rhythms, such as the sleep/wake cycle persist, despite the absence from sunlight, therefore demonstrating the existance of an endogenous pacemakers. However, these external cues are important as the clock in the study was not quite accurate and varied from day to day.
SIFFREE'S study is supported by further research from ASCHOFF AND WEVER, who placed participants in an undergound WWII bunker, thus isolating them from any external factors, and the results showed that this sleep/wake cycle operates despite an absence of external cues, thus supporting the idea of there being an endogenous pacemaker. Similarly, research by FOLKARD demonstrated that circadian rhythms can only be guided to a certain extent by internal factors, and external cues have an influence over such a cycle; it only took participants (who had been in a cave for 21 days) a few days to resynchronise their clocks to the avaliable external time cues, such as clocks and…