Biological Rhythms

• define biological rhythms

  • 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), the body’s biological clocks
  • exogenous zeitgebers (external), which are changes in the environment.
• define circadian rhythms

  24-hour circadian rhythm which is reset by levels of light.
• what is an example of a circadian rhythm?

  • 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 homeostasis.
• strength of circadian rhythms

  • 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.
• limitations of circadian rhythms (1)

  • 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.
• limitations of circadian rhythms (2)

  • 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. 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 and even small changes can make a difference. 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.
• define endogeneous 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.
  • 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.
• define exogenous zeitgebers

  • 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.
• strength of endogenous pacemakers and exogenous zeitgebers (2)

  • 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.
• strength of endogenous pacemakers and exogenous zeitgebers (2)

  • further research support for the role of exogenous zeitgebers. When Siffre 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.
• limitations of endogenous pacemakers and exogenous zeitgebers (1)

  • 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.
• limitations of endogenous pacemakers and exogenous zeitgebers (2)

  • 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.
• define infradian rhythms


  last longer than 24 hours and can be weekly, monthly or annually.
• give an example of an infradian rhythm

  • female menstrual cycle, which is regulated by hormones that either promote ovulation or stimulate the uterus for fertilisation.
  • Ovulation occurs roughly halfway through the cycle when oestrogen levels are at their highest, and usually lasts for 16-32 hours.
  • After the ovulatory phase, progesterone levels increase in preparation for the possible implantation of an embryo in the uterus.
  • usual menstrual cycle is around 28 days, there is considerable variation
• strength of infradian rhythms (1)

  • Research suggests that the menstrual cycle is, to some extent, governed by exogenous zeitgebers (external factors). Reinberg (1967) examined a woman who spent three months in a cave with only a small lamp to provide light. Reinberg noted that her menstrual cycle shortened from the usual 28 days to 25.7 days. This result suggests that the lack of light (an exogenous zeitgeber) in the cave affected her menstrual cycle, and therefore this demonstrates the effect of external factors on infradian rhythms.
• strengths of infradian rhythms (2)

  • exogenous zeitgebers can affect infradian rhythms. Russell et al. (1980) found that female menstrual cycles became synchronised with other females through odour exposure. In one study, sweat samples from one group of women were rubbed onto the upper lip of another group. Despite the fact that the two groups were separate, their menstrual cycles synchronised. This suggests that the synchronisation of menstrual cycles can be affected by pheromones, which have an effect on people nearby rather than on the person producing them.
• define ultradian rhythms

  last less than 24 hours
• give an example of an ultradian rhythm

  the sleep cycle
• give the stages of the sleep cycle

  1. Stages 1 and 2 are ‘light sleep’ stages. During these stages brainwave patterns become slower and more rhythmic, starting with alpha waves progress to theta waves.
  2. Stages 3 and 4 are ‘deep sleep’ or slow wave sleep stages, where it is difficult to wake someone up. This stage is associated with slower delta waves.
  3. Finally, Stage 5 is REM (or dream) sleep. Here is the body is paralysed (to stop the person acting out their dream) and brain activity resembles that of an awake person.
• strength of ultradian rhythms

  • flexibility of ultradian rhythms. Randy Gardener remained awake for 264 hours. While he experienced numerous problems he coped rather well with the massive sleep loss. After this experience, Randy slept for just 15 hours and over several nights he recovered only 25% of his lost sleep. Interestingly, he recovered 70% of Stage 4 sleep, 50% of his REM sleep, and very little of the other stages. These results highlight the large degree of flexibility in terms of the different stages within the sleep cycle and the variable nature of this ultradian rhythm.
• limitations of ultradian rhythms (1)

  • Individual Differences: The problem with studying sleep cycles is the differences observed in people, which make investigating patterns difficult. Tucker et al. (2007) found significant differences between participants in terms of the duration of each stage, particularly stages 3 and 4 (just before REM sleep). This demonstrates that there may be innate individual differences in ultradian rhythms, which means that it is worth focusing on these differences during investigations into sleep cycles.
• limitations of ultradian rhythms (2)

  • study was carried out in a controlled lab setting, which meant that the differences in the sleep patterns could not be attributed to situational factors, but only to biological differences between participants. While this study provide convincing support for the role of innate biological factors and ultradian rhythms, psychologists should examine other situational factors that may also play a role.
• limitations of ultradian rhythm (3)

  • the way in which such research is conducted may tell us little about ultradian rhythms in humans. When investigating sleep patterns, participants must be subjected to a specific level of control and be attached to monitors that measure such rhythms. This may be invasive for the participant, leading them to sleep in a way that does not represent their ordinary sleep cycle. This makes investigating ultradian rhythms, such as the sleep cycle, extremely difficult as their lack of ecological validity could lead to false conclusions being drawn.
• what are some effects of disruptions to biological rhythms?

  • Sleep Deprivation: People who work at night 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.
  • Social Consequences: 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.
• what are some effects of disruption to biological rhythms? (2)

  • 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. 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.