Occur once every 24 hours, most common examples are the sleep wake cycle, melatonin, testosterone and melatonin secretion, blood pressure and core body temperature. It is thought that main endogenous pacemaker (biological clock) involved is the supraciasmatic nucleus, a cluster of neutrons in the hypothalamus which cause the pineal gland to secrete melatonin which makes us feel sleepy. The sleep-wake cycle is also influenced by exogenous zeitgebers (external time givers) such as light, temperature or noise. The SCN responds to light but it can still control the rise and fall of melatonin suggesting that EPs and EZs both play a role but to different degrees.
Evidence to support the role of different factors in controlling circadian rhythms comes from Michel Siffre. He lived underground in a cave for 6 months with no cues from the sun.it was found that his sleep wake cycle settled into a free-running rhythm of 25 hours. Suggesting that EPs are important in controlling the sleep wake cycle as he continued to operate a regular sleep-wake cycle on a roughly daily rhythm. However, the study also suggests EZs such as light are important to the sleep-wake cycle. As Siffres rhythm was not in synchrony with days in the outside world (25 rather than 24) suggesting we need light in order to entrain our biological body clock. Nonetheless, as Siffres research was a case study of one individual, this study may not represent the sleep-wake cycle in everyone and the finidings may therefore not generalise to others.
However, Siffres findings are supported by Aschoff and Wever who studies the sleep wake cycles in participants living underground in a WWII bunker and found a similar free running cycle of around 25 years. Suggesting Siffres findings are reliable and can be generalised to other people (high population validity).
However an issue with making generalisations across all people is that it is a nomothetic approach which ignores individual...