Circadian Rhythms

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

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.

Evaluation

• Ralph et al. (1990) found that hamsters went to a 21 hour sleep wake cycle when given the SCN of hamsters who naturally had a 21 hour sleep wake cycle, showing internal pacemakers are important. This is an animal study and may not be generalisable to humans as internal pacemakers may not be as important in humans.
  

• 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.
  

• 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.
  

• The sleep wake cycle may be affected by endogenous pacemakers such as light showing other factors are involved in the regulation of sleep. Boivin et al. (1996) found that bright artificial light advanced the sleep wake cycle by five hours showing endogenous pacemakers also have an effect as well as internal pacemakers.
  

• 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.

Internal Pacemakers & Exogenous Zeitgebers

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 affected by the environment. The most important endogenous pacemaker is the suprachiasmatic nucleus (SCN) controls other biological rhythms, as it links to other areas of the brain responsible for sleep and arousal. The SCN is in the hypothalamus and receives information from the optic nerve. It can detect how light or dark it is in the surrounding environment. When it is dark the SCN stimulates the pineal gland, causing it to produce more melatonin. The SCN also regulates body temperature, lowering it when it gets dark.

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.

How they work together: 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. I.e.

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.
  

• 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.
  

• However, Siffre’s study was only completed on one individual and therefore may not be representative of the sleep-wake cycle.
  

• 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) support longer sleep-wake cycle than 24 hours.
  

• Research on the influence of external zeitgebers has been completed on animals so generalising to humans is difficult.
  

• People living in the Arctic circle have winter days with little daylight, if circadian rhythm was purely linked to an exogenous zeitgeber of light, they would sleep most of the day.
  

• Evidence over simplifies the sleep-wake cycle’s reliance on light, whereas the role of neurotransmitters can play a more significant role than credited.

Past Paper Questions

• Identify the type of bodily rhythm that Siffre (1972) was researching. (1) June 2016

• Explain one weakness of research conducted into the sleep-wake cycle. (2) June 2016

3 Markers

• Samadhi lives in Norway and she finds her sleep-wake cycle changes according to the season. During the winter months Samadhi goes to bed earlier and wakes up later compared to during the summer months. Describe the role of internal pacemakers on Samadhi’s sleep-wake cycle. (3) January 2018

• Gabriella is going to another country for a holiday; it is eight hours behind the time of her home country. She is concerned she will want to sleep during the day for the first few days of her holiday. Gabriella plans to eat her usual evening meal when she arrives, even though it will be early morning. Her friend suggests she uses external zeitgebers to help regulate her sleep-wake cycle. Describe how Gabriella can use external zeitgebers to regulate her sleep-wake cycle while she is on holiday. (3) October 2018

4 Markers

• Explain one strength and one weakness of Gabriella using external zeitgebers to regulate her sleep-wake cycle. (4) October 2018

• Explain, using research, why the regulation of an internal body clock could change. (4) June 2016

• Explain one strength and one weakness of the role of internal pacemakers on the sleep-wake cycle. (4) January 2018

8 Markers

• Discuss, using the role of internal pacemakers, why Doris finds it hard to sleep when she gets home after the night shifts. You must refer to the context in your answer. (8) October 2019

• Assess the importance of the role of internal pacemakers (body clock) in the regulation of sleep. (8) October 2017

12 Markers

• Evaluate the role of internal pacemakers (body clock) and external zeitgebers in the regulation of the sleep-wake cycle. (12) October 2016