Endogenous Pacemakers & Exogenous Zeitgebers

What do endogenous pacemakers and exogenous zeitgebers do?

• they regulate biological rhythms

• they interact with one another to control and fine-tune biological rhythms and therefore it is necessary to consider these concepts together

What are exogenous zeitgebers?

Environmental events that are responsible for resetting the biological clock of an organism

• social cues such as meal time

• light which is responsible for resetting the body clock each day, keeping it on a 24 hour cycle

What are endogenous pacemakers?

• internal mechanisms that govern biological rhythms (e.g. sleep-wake cycle)

What does the suprachiasmatic nucleus do?

• the most important endogenous pacemaker is the suprachiasmatic nucleus which is found in the hypothalamus

• SCN = master clock: endogenous pacemaker

• SCN = receives information about light (exogenous zeitgeber) via the optic nerve

  • SCN sends signal to pineal gland

  • increased melatonin at night

• the SCN and the pineal gland work together as endogenous pacemakers however their activity is responsive to the external cue of light (exogenous zeitgeber)

AO3 - research support for endogenous pacemakers

P = there is research support for the importance of endogenous pacemakers

EX = in particular the suprachiasmatic nucleus (SCN), in relation to the sleep-wake cycle

EV = Morgan (1995), 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 who subsequently displayed the same abnormal circadian rhythm of 20 hours, showing that the transplanted SCN had imposed its pattern onto the hamsters

C = however, there is the problem with extrapolation 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

LB = therefore while this research demonstrates the significance of the SCN and how endogenous pacemakers impact biological rhytms (e.g. the sleep-wake cycle), research carried out on animals may be unable to explain the role of endogenous pacemakers on the sleep-wake cycle in humans

AO3 - research support for the role of exogenous zeitgebers

P = there is research support for the role of exogenous zeitgebers on the sleep-wake cycle

Ex = in particular the influence of light

EV = when Michael Siffre returned from an underground stay with no clocks or light, he believed the data to be a month earlier than it was which demonstrates the importance of the exogenous zeitgeber (light) on the sleep-wake cycle

C = however, Siffre’s case study has been the subject of criticism. As the research and sole participant in his case study, there are several issues with the potential researcher bias and a lack of generalisability to the wider population. Individual differences has been seen to be an important factor when it comes to the sleep wake cycle

LB = therefore while this result suggests that the 24 hour sleep-wake cycle was increased by the lack of exogenous zeitgebers, showing the impact of such factors o the bodily rhythms, these findings should be treated with caution; we are unable to conclude whether the role of exogenous zeitgebers effects the sleep-wake cycle in a wider population on the basis of this case study alone as an ideographic approach

AO3 - reductionist

P = In addition to this, it could be argued that the idea of endogenous pacemakers and exogenous zeitgebers is reductionist.

EX = Despite all of the research to support the role of the external and internal cues, it could be argued that this idea oversimplifies a complex human phenomena.

EV = For example, critics say that the sleep-wake cycle is influenced by other people and social norms, like how sleep occurs when its 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.

EXT = Furthermore, the influence of exogenous zeitgebers may be overstated. Miles et al (1977) worked with a man who was blind from birth with a circadian rhythm of 24.9 hours. Despite exposure to social cues, his sleep-wake cycle could not be adjusted and consequently he had to take sedatives at night and stimulants in the morning to keep pace with the 24-hour world.

LB = Therefore, the research fails to consider the other widely divergent viewpoints and this suggests that our understanding of the effect of endogenous pacemakers and exogenous zeitgebers on the sleep-wake cycle is limited.