Specification: Circadian rhythms. The effect of endogenous pacemakers and exogenous zeitgebers on the sleep/wake cycle.
Biological rhythms are cyclical patterns within biological systems that have evolved in response to environmental influences, e.g. day and night. There are two key factors that govern biological rhythms: endogenous pacemakers (internal factors), the body’s biological clocks, and exogenous zeitgebers (external factors), which are changes in the environment.
Exam Hint: For each of the biological rhythms (circadian, infradian and ultradian) it is important you that you can define the term and provide at least one example.
Circadian rhythms
One biological rhythm is the 24-hour circadian rhythm (often known as the ‘body clock’), which is reset by levels of light. The word circadian is from the Latin ‘circa’ which means ‘about’, and ‘dian’, which means ‘day’.
The sleep-wake cycle is an example of a circadian rhythm, which dictates when humans and animals should be asleep and awake. Light provides the primary input to this system, acting as the external cue for sleeping or waking. Light is first detected by the eye, which then sends messages concerning the level of brightness to the suprachiasmatic nuclei (SCN). The SCN then uses this information to coordinate the activity of the entire circadian system. Sleeping and wakefulness are not determined by the circadian rhythm alone, but also by homoeostasis. When an individual has been awake for a long time, homeostasis tells the body that there is a need for sleep because of energy consumption. This homeostatic drive for sleep increases throughout the day, reaching its maximum in the late evening, when most people fall asleep.
Body temperature is another circadian rhythm. Human body temperature is at its lowest in the early hours of the morning (36oC at 4:30 am) and at its highest in the early evening (38oC at 6 pm). Sleep typically occurs when the core temperature starts to drop, and the body temperature starts to rise towards the end of a sleep cycle promoting feelings of alertness first thing in the morning.
Evaluating circadian rhythms
Research Support: Research has been conducted to investigate circadian rhythms and the effect of external cues like light on this system. Siffre (1975) found that the absence of external cues significantly altered his circadian rhythm: When he returned from an underground stay with no clocks or light, he believed the date to be a month earlier than it was. This suggests that his 24-hour sleep-wake cycle was increased by the lack of external cues, making him believe one day was longer than it was, and leading to his thinking that fewer days had passed. o Siffre’s case study has been the subject of criticism. As the researcher and sole participant in his case study, there are severe issues with generalisability. However, further research by Aschoff & Weber (1962) provides additional support for Siffre’s findings. Aschoff & Weber studied participants living in a bunker. The bunker had no windows and only artificial light, which the participants were free to turn on and off as they pleased. Aschoff & Weber found that the participants settled into a longer sleep/wake cycle of between 25-27 hours. These results, along with Siffre’s findings, suggest that humans use natural light (exogenous zeitgebers) to regulate a 24-hour circadian sleep-wake cycle, demonstrating the importance of light for this circadian rhythm.
Individual Differences: However, it is important to note the differences between individuals when it comes to circadian cycles. Duffy et al. (2001) found that ‘morning people’ prefer to rise and go to bed early (about 6 am and 10 pm) whereas ‘evening people’ prefer to wake and go to bed later (about 10 am and 1 am). This demonstrates that there may be innate individual differences in circadian rhythms, which suggests that researchers should focus on these differences during investigations.
Additionally, it has been suggested that temperature may be more important than light in determining circadian rhythms. Buhr et al. (2010) found that fluctuations in temperature set the timing of cells in the body and caused tissues and organs to become active or inactive. Buhr claimed that information about light levels is transformed into neural messages that set the body’s temperature. Body temperature fluctuates on a 24-hour circadian rhythm and even small changes in it can send a powerful signal to our body clocks. This shows that circadian rhythms are controlled and affected by several different factors, and suggests that a more holistic approach to research might be preferable.
Biological rhythms are regulated by endogenous pacemakers, which are the body’s internal biological clocks, and exogenous zeitgebers, which are external cues, including light, that help to regulate the internal biological clocks.
Exam Hint: It’s important to note that endogenous pacemakers and exogenous zeitgebers interact with one another to control and fine-tune biological rhythms and therefore it is necessary to consider these concepts together.
Endogenous pacemakers
Endogenous pacemakers are internal mechanisms that govern biological rhythms, in particular, the circadian sleep-wake cycle. Although endogenous pacemakers are internal biological clocks, they can be altered and affected by the environment. For example, although the circadian sleep-wave cycle will continue to function without natural cues from light, research suggests that light is required to reset the cycle every 24 hours. (See Siffre and Aschoff & Weber)
The most important endogenous pacemaker is the suprachiasmatic nucleus, which is closely linked to the pineal gland, both of which are influential in maintaining the circadian sleep/wake cycle.
The suprachiasmatic nucleus (SCN), which lies in the hypothalamus, is the main endogenous pacemaker (or master clock). It controls other biological rhythms, as it links to other areas of the brain responsible for sleep and arousal. The SCN also receives information about light levels (an exogenous zeitgeber) from the optic nerve, which sets the circadian rhythm so that it is in synchronisation with the outside world, e.g. day and night.
The SNC sends signals to the pineal gland, which leads to an increase in the production of melatonin at night, helping to induce sleep. The SCN and pineal glands work together as endogenous pacemakers; however, their activity is responsive to the external cue of light. Put simply:
Exogenous zeitgebers
As outlined above, exogenous zeitgebers influence biological rhythms: these can be described as environmental events that are responsible for resetting the biological clock of an organism. They can include social cues such as meal times and social activities, but the most important zeitgeber is light, which is responsible for resetting the body clock each day, keeping it on a 24-hour cycle.
The SNC contains receptors that are sensitive to light and this external cue is used to synchronise the body’s internal organs and glands. Melanopsin, which is a protein in the eye, is sensitive to light and carries the signals to the SCN to set the 24-hour daily body cycle. In addition, social cues, such as mealtimes, can also act as zeitgebers and humans can compensate for the lack of natural light, by using social cues instead.
Evaluating endogenous pacemakers and exogenous zeitgebers
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.
There is research support for the role of melanopsin. 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.
There is further research support for the role of exogenous zeitgebers. When Siffre (see above) returned from an underground stay with no clocks or light, he believed the date to be a month earlier than it was. This suggests that his 24-hour sleep-wake cycle was increased by the lack of external cues, making him believe one day was longer than it was. This highlights the impact of external factors on bodily 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 behaviourist 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.
Extension: disruption of biological rhythms
People who work at night (shift work) often experience symptoms similar to jet lag. This is because the person’s work schedule (exogenous zeitgeber) is at odds with their circadian sleep-wave cycle, which is governed by powerful biological factors (endogenous pacemakers). Not surprisingly, people who work shift work often feel sleepy at work and suffer from insomnia at home.
There are numerous consequences for people who work shifts, including:
Sleep Deprivation: People who work at night and have to sleep during the day often experience difficulties in sleeping. This is because their biological clocks (endogenous pacemakers) do not adjust completely. Furthermore, the daytime is associated with significantly more noise and other disturbances that can also affect sleep. Research suggests that daytime sleep is shorter than night-time sleep. Tilley and Wilkinson (1982) suggest that REM is particularly affected and this reduction in sleep results in sleep deprivation, which produces lower levels of energy and reduces alertness during the night time (awake period).
Heart Disease: There is a relationship between shift work and heart disease. Knutsson (1986) found that people who worked shift patterns for more than 15 years were significantly more likely to develop heart disease. This research highlights the negative health consequences of disrupting biological rhythms, in particular the sleep-wake cycle. However, it is worth noting that these findings are purely correlational and while the findings might indicate a link between the disruption of biological rhythms and heart disease, other factors may also play a significant role. For example, it may be that jobs that require night time working are inherently more stressful and it is the stress that is the major factor and not the shift work.
Social Consequences: Another issue that people who work shift patterns experience is social disruption. People who work hours that are at odds with the hours worked by their family and friends find it difficult to spend quality time with significant others.
Practical Applications: While the research above highlights a series of negative consequences associated with disrupting biological rhythms, Czeisler et al. (1982) used research on shift work to improve the health and performance of shift workers. They found that by using a phase system to make shift changes slower, workers reported increased satisfaction and increase productivity. This suggests that the negative impact of disrupting biological rhythms can be overcome by slowly introduce people to night work.
Possible exam questions
Outline one or more examples of circadian rhythms. (4 marks)
Outline what research into circadian rhythms has found. (4 marks)
Outline and evaluate circadian rhythms. (16 marks)
What is meant by the terms endogenous pacemakers and exogenous zeitgebers. (4 marks)
John is an electrician and has just started a new job where he works shifts. He works 4 day shifts, then has 4 days off, and then works 4 night shifts. After working night shifts, John finds it difficult to sleep during the day and becomes very frustrated. Using your knowledge of endogenous pacemakers and exogenous zeitgebers, explain John’s experiences. (4 marks)
Outline and evaluate the effect of endogenous pacemakers and exogenous zeitgebers on the sleep-wake cycle. (16 marks)
