Black: AO1 - Description
Blue: AO2 - Evaluation - studies
Red: AO2 - Evaluation - evaluative points/IDAs
Control of infradian rhythms - EPs and EZs
Infradian rhythms are biological rhythms with a cycle length of more than 24 hours - such as the menstrual cycle, with a rhythm length of a month, or Seasonal Affective Disorder (SAD), with seasonal fluctuation. SAD is characterised by depression experienced in the winter months, which then disappears during the summer. There are two main endogenous pacemakers that control SAD: the hormone melatonin and the neurotransmitter serotonin. This two chemicals are normally in a balanced equilibrium, but when an increase in melatonin occurs, leading to a fall in serotonin levels, depression can develop. Light is a key exogenous zeitgeber in this cycle. During the winter months, when there is much less sunlight, the pineal gland will produce more melatonin, leading to a fall in serotonin levels that causes depression. This explains the winter onset and summer disappearance of SAD.
Evidence to support the serotonin-melatonin hypothesis of SAD comes from the successful real-world application of phototherapy in its treatment. Daily exposure to artificially heightened light levels using a light box has been shown to help alleviate symptoms of SAD. This, as well as being a valuable application of the theory, supports the idea of low light exposure causing SAD's melatonin rise and serotonin fall.
Further supporting evidence comes from a study by Eastman, who found that in SAD patients, a reduction in symptom severity was much more likely when exposed daily to bright morning light
rather than dim evening light or a placebo. Again, this supports the concept of a sunlight deficiency being responsible for SAD - exposure to the bright lights triggered the release of serotonin, which would help restore a healthy serotonin/melatonin equilibrium.
However, Murphy measured the serotonin and melatonin of a sample of SAD sufferers and a control group of non-sufferers hourly over a few days, and found no significant differences between the groups for the levels of either chemical. This challenges the serotonin-melatonin hypothesis, suggesting that factors other than chemical levels are present in the development of SAD.
Biological reductionism is a key issue with this theory. During winter, many people have much lower levels of social contact and physical activity - both of these factors could play a role in the winter onset of SAD. It is overly simplistic to suggest that the disorder is purely a result of biological factors, when social factors could play just as important a role in explaining its winter onset.
The menstrual cycle is another infradian rhythm with an average cycle length of 28 days. It is regulated by the hormones progesterone and oestrogen, produced by the pituitary gland and the ovaries. As the cycle beginds, both hormone levels are low during menstruation, but a surge in oestrogen levels triggers ovulation. After this, progesterone levels steadily increase over two weeks to maintain the uterine lining to prepare for a pregnancy. If there is no fertilisation of the egg after two weeks, a drop in both hormone levels triggers menstruation, restarting the cycle.
Research has demonstrated that the pheromones of other women are an exogenous zeitgeber that affect hormone levels, influencing the menstrual cycle. Russell used a single blind trial, applying cotton pads containing traces of pheromones from the "odour donor" to participants, and found that the participants' menstrual cycles synchronised with those of the donor. This supports the concept of pheromones as an EZ that can influence this infradian rhythm.
Through its use of a single-blind trial and a control group to help establish cause and effect, this study uses fairly scientific methodology to gather data. This lends credibility to the results - due to these controls, it has high replicability, meaning it can be repeated with similar results on different sample groups, helping establish and generalise a conclusion.
However, a study by Yang and Schank challenged this conclusion. Studying 186 pairs of Chinese women who lived together, controlling for other studies' methodological errors, no synchronisation of cycles was found other than by pure chance. This suggests that pheromones are not an EZ that affects this biological rhythm, and the menstrual cycle is only controlled by hormonal EPs.
However, a key issue with this study is that despite a large sample size, it lacks generalisability, and is culturally biased when attempting to apply it. Studying only Chinese women and then attempting to globally apply the results is imposing an etic construct on all cultures - especially as high levels of environmental pollution in China could interfere with pheromone transmission, making Chinese women particularly unrepresentative of the global population. This attempt at global generalisation imposes an etic as it marginalises potential differences between ethnicities and cultures that could mean the results do not apply cross-culturally.
Conclusion
In conclusion, while there is evidence to support the role of exogenous zeitgebers in infradian rhythm control, both SAD and the menstrual cycle seem to be predominantly regulated by endogenous pacemakers. However, evidence has shown that free will can affect biological and biochemical systems, (Born et al, 1999, early wakers by choice have higher ACTH levels in the blood), so suggesting that SAD is entirely a result of uncontrollable biological processes is overly deterministic. Similarly, the true mechanism that controls infradian rhythms is likely to be a result of both biological and environmental factors - it is too reductionist to single out either type of factor as being entirely responsible.
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