Biological Rhythms - The Infradian Rhythm Flashcards
Infradian Rhythms
Infradian rhythms are biological cycles that last longer than 24 hours and can occur over a period of days, weeks, or seasons.
Examples of Infradian Rhythms
- The Menstrual Cycle
- Seasonal Affective Disorder (SAD)
- The Menstrual Cycle
A monthly infradian rhythm is the 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 last for 16-32 hours. After the ovulatory phase, the progesterone levels increase in preparation for the possible implantation of an embryo in the uterus. It is also important to note that although the usual menstrual cycle is around 28 days, there is considerable variation, with some women experiencing a short cycle of 23 days and others experiencing a longer cycle of up to 36 days.
- Seasonal Affective Disorder (SAD)
Another example of an infradian rhythm is related to the seasons. Research has found seasonal variation in mood, where some people become depressed in the winter, which is known as seasonal affective disorder (SAD). SAD is an infradian rhythm that is governed by a yearly cycle. Psychologists claim that melatonin, which is secreted by the pineal gland during the night, is partly responsible. The lack of light during the winter months results in a longer period of melatonin secretion, which is linked to the depressive symptoms.
Strength
Point: Research suggests that the menstrual cycle is influenced not only by endogenous pacemakers but also by exogenous zeitgebers, such as light exposure.
Evidence: Reinberg studied a woman who spent three months in a cave with only a small lamp as a light source. During this time, her menstrual cycle shortened from the typical 28 days to 25.7 days, suggesting that the reduced exposure to natural light had shortened her cycle.
Justification: This finding supports the idea that external factors, in this case light exposure, can regulate infradian rhythms. It aligns with other research showing that changes in light exposure affect melatonin production, which in turn influences hormone regulation and menstrual cycles.
Implication: Understanding the role of exogenous zeitgebers in infradian rhythms has strong practical applications, such as in managing menstrual regularities in individuals exposed to unusual light conditions, e.g., shift workers or astronauts.
Counterargument: Although Reinberg’s study provided valuable insight into the effects of exogenous zeitgebers on infradian rhythms, it is limited in terms of generalisability due to its idiographic nature.
Evidence: As a case study, the research focused only on one woman, meaning the findings may not be representative of the wider population. Individual differences, such as genetic factors, hormonal variations and lifestyle, could influence menstrual cycles in ways that were unique to this participant. Additionally, other confounding variables, such as changes in diet, stress levels, or disrupted sleep patterns from living in isolation, may have inadvertently contributed to the alterations in her cycle rather than light exposure alone.
Justification: This means that the study lacks experimental control, making it difficult to establish a direct causal relationship between light and menstrual cycle changes.
Implication: Consequently, the study lacks population validity because the findings cannot confidently apply to all women. To strengthen the validity of this research, further studies with larger, more diverse samples and greater experimental control are needed to determine whether light exposure consistently influences infradian rhythms across different individuals.
Strength
Point: There is strong research support for the role of melatonin in the development of Seasonal Affective Disorder (SAD), highlighting the influence of exogenous zeitgebers, such as light, on infradian rhythms.
Evidence: Terman found that the prevalence of SAD is significantly higher in northern regions, where winter nights are longer. For instance, around 10% of people in New Hampshire (a northern US state) experience SAD, compared to only 2% in southern Florida, where daylight hours remain more consistent throughout the year.
Justification: This supports the idea that reduced light exposure in winter increases melatonin production, disrupting infradian rhythms and leading to depressive symptoms. The findings align with the biological explanation of SAD, which suggests that excess melatonin secretion during prolonged darkness affects serotonin levels, contributing to low mood and lethargy.
Implication: These findings have fundamental real-world applications, particularly in the treatment of SAD. Light therapy (phototherapy), which mimics natural daylight to regulate melatonin levels, has been developed as an effective treatment for SAD sufferers, therefore showcasing its strong impact.
Counterargument: However, while Terman’s study highlights a clear relationship between light exposure and SAD, it is important to acknowledge that multiple factors beyond light may also contribute to the development of the disorder.
Evidence: Factors such as genetics, lifestyle and psychological vulnerability have been shown to play a key role in the development of SAD. For instance, individuals with a family history of depression are more likely to experience SAD, suggesting a genetic predisposition. Additionally, certain lifestyle factors, such as lack of physical activity or poor diet, can exacerbate depressive symptoms during winter months. Psychological factors may involve high levels of stress or negative thinking patterns, which may increase susceptibility to SAD.
Justification: This indicates that the causes of SAD could be multifactorial, meaning that it may not be solely influenced by light exposure and melatonin regulation. The interaction between biological, psychological, and environmental factors creates a more complex picture of SAD, inferring that treatment approaches based solely on the influence of light exposure on the development of SAD, may not be sufficient.
Implication: Therefore, a more holistic approach is needed when studying and treating SAD. In practice this means that clinicians should consider not only light therapy but also incorporate lifestyle modifications, e.g., improved sleep hygiene and increased physical activity, as well as psychological treatments, e.g., Cognitive Behavioural Therapy. By addressing the interdependent nature of the disorder, a more comprehensive and perhaps personalised treatment plan can be developed, improving overall outcomes for individuals with SAD.
Strength
Point: There is further evidence suggesting that exogenous zeitgebers, such as pheromones, can influence infradian rhythms, particularly the menstrual cycle.
Evidence: Russell et al. conducted a study where sweat samples from one group of women were rubbed onto the upper lips of another group. Despite the two groups being separate, their menstrual cycles synchronised. This finding suggests that pheromones, an external cue, have the power to influence menstrual cycle synchronisation, even though the women involved were not physically in close proximity to one another.
Justification: This study challenges the traditional view that infradian rhythms, such as the menstrual cycle, are solely governed by internal biological processes (endogenous pacemakers). Instead, it highlights the role of external factors (exogenous zeitgebers) in influencing these rhythms. The ability of pheromones to synchronise menstrual cycles indicates that external environmental factors can have a substantial impact on biological rhythms.
Implication: This underscores the need for a more holistic approach to understanding infradian rhythms. It suggests that endogenous and exogenous influences must be considered together to provide a more exhaustive understanding of infradian rhythms. This could have immense practical applications in healthcare and menstrual health management.
Weakness
Point: One issue with research into infradian rhythms is that ethical concerns may arise due to the methodology used to study them.
Evidence: Russell et al. applied sweat samples from one group of women onto the upper lips of another, which could be considered invasive and unhygienic. Similarly, McClintock studied menstrual synchrony by closely tracking women’s cycles over long periods, raising privacy concerns regarding personal biological data. Research into SAD, such as Terman, involves exposing participants to artificial light therapy, which, while effective, can have unpreventable side effects, such as headaches and nausea, raising ethical considerations about participant distress.
Justification: These studies involve procedures that could cause discomfort or intrude on personal privacy, which may lead to participant withdrawal or distress, especially if they are not fully aware of the procedure’s nature beforehand. Additionally, in long-term studies, informed consent must be maintained throughout, ensuring that participants remain fully aware of what is being tracked so they don’t feel uncomfortable sharing sensitive biological data.
Implication: Ethical concerns may limit future research on infradian rhythms, as stricter ethical guidelines are now required. This may reduce the feasibility of replicating older studies, affecting the reliability and validity of past findings. Consequently, alternative, less invasive methods may be required to study the impact of exogenous zeitgebers on infradian rhythms effectively.
