Circadian Rhythms | Psychology Hacked

Specification

The role of internal pacemakers (body clock) and external zietgebers in the regulation of the circadian sleep-wake cycle.
Research into the circadian sleep-wake cycle.

The Sleep-Wake Cycle

The sleep-wake cycle, also known as the circadian rhythm, is the 24-hour cycle of physiological processes that occur in the body. This cycle is regulated by two main mechanisms: endogenous pacemakers and exogenous zeitgebers.

Endogenous pacemakers are internal biological clocks that are located in the hypothalamus of the brain, specifically in a small region called the suprachiasmatic nucleus (SCN). The SCN receives signals from the eyes about the presence or absence of light, and uses this information to regulate the production of hormones such as melatonin, which helps to regulate the sleep-wake cycle.

Exogenous zeitgebers are external cues that help to synchronise the body’s internal clock with the external environment. The most important zeitgeber is light, which is why the SCN receives signals from the eyes about the presence or absence of light. Other examples of zeitgebers include temperature, social cues, and meal times.

Together, these endogenous pacemakers and exogenous zeitgebers work to coordinate the sleep-wake cycle and other physiological processes with the external environment. For example, during the day, exposure to light will suppress the production of melatonin, making us feel alert and awake, while at night, the absence of light will increase melatonin production, making us feel sleepy. Additionally, the sleep-wake cycle is also influenced by various internal biological processes like the release of hormones, temperature changes and hunger levels, which all together synchronize to keep the body’s internal clock in sync with the external environment.

Michel Siffre

Michel Siffre was a French geologist and speleologist who conducted a series of pioneering studies on the effects of isolation from external time cues, such as light and social cues, on the circadian rhythms of the human body. One of his most notable studies was conducted in 1962, in which he spent six months living in a subterranean cave.

During his time in the cave, Siffre had no access to natural light or any other external time cues. He recorded his sleep-wake cycle, as well as his mood, physical activity, and other physiological measures. Siffre’s study showed that without the presence of external cues, such as light, his sleep-wake cycle became desynchronised from the 24-hour day-night cycle. Siffre slept and woke at different times each day, and his sleep patterns were irregular.

Siffre observed that without the presence of external cues, his internal clocks was not able to keep a consistent 24-hour rhythm. This indicates that his internal clocks was not able to generate a consistent sleep-wake cycle in the absence of exogenous zeitgebers.

Siffre’s research supports the idea that external cues, such as light, are important for synchronising the body’s internal clock with the external environment, and that without these cues, the body’s internal clock is not able to generate a consistent sleep-wake cycle.

Evaluation

The importance of the SCN has been demonstrated in research. Morgan (1955) bred hamsters so that they had circadian rhythms of 20 hours rather than 24. SCN neurons from these abnormal hamsters were transplanted into the brains of normal hamsters, which subsequently displayed the same abnormal circadian rhythm of 20 hours, showing that the transplanted SCN had imposed its pattern onto the hamsters. This research demonstrates the significance of the SCN and how endogenous pacemakers are important for biological circadian rhythms.
However, this research is flawed because of its use of hamsters. Humans would respond very differently to manipulations of their biological rhythms, not only because we are different biologically, but also because of the vast differences between environmental contexts. This makes research carried out on other animals unable to explain the role of endogenous pacemakers in the biological processes of humans.
Skene and Arendt (2007) claimed that the majority of blind people who still have some light perception have normal circadian rhythms whereas those without any light perception show abnormal circadian rhythms. This demonstrates the importance of exogenous zeitgebers as a biological mechanism and their impact on biological circadian rhythms.
Despite all the research support for the role of endogenous pacemakers and exogenous zeitgebers, the argument could still be considered biologically reductionist. For example, the social approach would suggest that bodily rhythms are influenced by other people and social norms, i.e. sleep occurs when it is dark because that is the social norm and it wouldn’t be socially acceptable for a person to conduct their daily routines during the night. The research discussed here could be criticised for being reductionist as it only considers a singular biological mechanism and fails to consider the other widely divergent viewpoints.
Aschoff and Weber (1963) conducted a study in which they examined the circadian rhythms of people living in isolation from external time cues, such as light and social cues. They found that the participants’ sleep-wake cycle gradually lengthened over time, with a period of approximately 25 hours. This suggests that the body’s internal clock has a natural period that is slightly longer than 24 hours.
Even though the people in the Arctic Circle experience extreme variations in daylight hours, they still maintain a relatively consistent sleep-wake cycle throughout the year. Studies of people living in the Arctic Circle have shown that their sleep-wake cycle is influenced by the changes in daylight hours, but it is also influenced by their internal biological clocks. For example, during the summer months when there is continuous daylight, the body’s internal clock is still able to regulate the production of melatonin, which helps to regulate the sleep-wake cycle. Similarly, during the winter months when there is continuous darkness, the body’s internal clock is still able to regulate the sleep-wake cycle, despite the absence of external cues such as light. This suggests that the body’s internal clock, or endogenous pacemaker, is able to regulate the sleep-wake cycle independently of external cues such as light.