Endogenous Pacemakers and Exogenous Zeitgebers

• Endogenous Pacemakers = Internal bodily regulators of biological rhythms, affecting such biological rhythms to conform to certain cyclical periods e.g. one cycle every 24 hours for circadian rhythms.
• The suprachiasmatic nucleus (SCN) receives information about day light and day length from the eyes
• The SCN processes information and triggers different rates of release of melatonin from the pineal gland. Increased melatonin release triggers decreased serotonin production, creating feelings of sleepiness during the night time, where there is little exposure to light.
• during the daytime, when there is high exposure to light, the SCN triggers the pineal gland to release less melatonin over a longer period of time, resulting in increased serotonin production, creating feelings of wakefulness.
• Ralph et al extracted SCN cells from hamsters which showed abnormal sleep-wake cycles, and inserted these cells into healthy hamster foetuses. The researchers found that “the restored rhythms always showed similarity to donor, regardless of the genotype of the host. The basic period of the circadian rhythm therefore is determined by cells of the suprachiasmatic region.
• Exogenous Zeitgebers = External environmental changes, affecting or ‘entraining’ biological rhythms to conform to certain cyclical time periods.
• Social cues are examples of exogenous zeitgebers which entrain biological rhythms. These include set meal times and bed times, which signify when to wake up and when to fall asleep. This means that in order to avoid jet lag, it is useful to accustom yourself to the set sleeping and eating times of your destination, to avoid desynchronisation of an already ‘pre-set’ circadian rhythm.
• There are considerable ethical issues with the use of animals in such research, particularly if they are deliberately put in harms way as was the case with russel et al, thus breaching the BPS ethical guideline of protection from psychological and physical harm. Although this does not impact upon the utility or validity of the findings, a cost-benefit analyses would have to be conducted to assess whether such ethical costs outweigh the benefits of an improved understanding of exogenous zeitgebers and endogenous pacemakers.
• A second problem occurs with the limitations of generalising findings from animal studies to humans, particularly due to differences in physiology and the number/types of circadian rhythms, thus limiting the ecological validity of such findings.