Discuss the role of endogenous pacemakers in at least one biological rhythm

Exogenous zeitgebers are external stimuli which may be rhythmic and act like a prompt, affecting internal rhythms. For example, day length is the dominant EZ as proved by Miles’ case study, where light is the dominant cue and the blind man was unable to have a normal circadian rhythm, but proved that it can be overcome. Luce and Segal disputed the fact that light is the dominant cue. Studying the arctic circle where light is either constantly available or never, found that the participants had a sleep cycle for seven hours which was based on social cues and was therefore argued to be the dominant EZ. It is adaptive to have biorhythms governed by external cues to be in tune with the environment. Both variables are not distinct, but interact to regulate the timing of biorhythms. Some cases show only one is prominent, for example hibernation, however, it is common that we adapt to the environment, but individual differences always dictate the balance and affect.

Circadian rhythms are biological rhythms that occur within 24 hours. The most common circadian rhythm is the sleep-wake cycle. It is normally governed by EZ’s such as light, but is inherently an internal body clock. Aschoff and Wever showed that without any EZs, participants followed a circadian rhythm. This proves that even without the affect of external cues, the sleep-wake cycle is an internal body clock. Siffre showed the malleability of the circadian rhythm and the vast importance of EP by living for six months in a cave with very few EZ’s which changed the rhythm from the standard 25 hour to 48 hours. Siffre then hypothesised that the main EZ in the sleep-wake cycle was light and it’s affect on melatonin, and more importantly EZ’s in general are the most significant variable in manipulating biorhythms, contradicting the previous research on SCN activity.

However, it is an idiographic study that lacks ecological validity, and not all external cues were removed, for example the role of artificial light could have easily manipulated the biorhythm and misplaced for the role of other variables. We are therefore unable to generalise the findings. It does however have good evidence for EZ’s influence on EP’s, proving they are not distinct and separate drives, and this must be considered when attempting to separate such variables. Nevertheless, there is much debate about which variable has the most importance. One study that found SCN damage in rats found their circadian rhythms were completely eliminated. However, you cannot sufficiently generalise from comparable studies.

Endogenous rhythms are limited in control, and research into this seems to contradict Siffre’s ability to manipulate the cycle to such an extent. In Folkard’s study, participants were in an isolation unit and given a clock to refer to. When this was changed to 22 hours a day without them knowing, only one participant changed their cycle to the 22 hours. This is evidence that EPs can become free running if all EZs are removed and the role of individual differences.

There is no clear evidence for which exogenous zeitgebers or endogenous pacemakers have affects on the sleep-wake cycle and the various biological rhythms, often due to conflicting research and heavy methodological issues, but some clear patterns do emerge from even the contradictory studies, for example, the role of the SCN in endogenous pacemakers seems to be the master clock and challenging scientific research has supported this.