Infradian Rhythms (AQA A-Level Psychology): Revision Notes
Infradian Rhythms
Infradian rhythms are biological rhythms that take longer than 24 hours to complete one full cycle. Unlike circadian rhythms which occur daily, infradian rhythms happen less frequently - sometimes weekly, monthly, or even seasonally.
Infradian rhythms are distinct from other biological rhythms because of their extended duration. While circadian rhythms reset daily, infradian rhythms can take weeks, months, or even entire seasons to complete their cycle.
Key characteristics
Infradian rhythms are primarily controlled by the body's endocrine system rather than purely by hormonal release. However, external factors called zeitgebers (environmental cues) can also influence these rhythms. Examples include light exposure and chemical signals such as pheromones.
What are zeitgebers? Zeitgebers are environmental cues that help regulate biological rhythms. The term comes from German, meaning "time givers," and includes factors like light, temperature, and social interactions.
Examples of infradian rhythms
The menstrual cycle
The most commonly studied infradian rhythm is the human menstrual cycle, which typically occurs over approximately 28 days. This cycle involves complex hormonal changes that prepare the female reproductive system for potential pregnancy.
Hibernation patterns
Some animals exhibit seasonal infradian rhythms, such as hibernation cycles that occur annually. These patterns help organisms survive harsh environmental conditions by conserving energy during difficult periods.
Example: Bear Hibernation
Bears demonstrate a clear infradian rhythm through their annual hibernation cycle:
- Preparation phase (late summer/autumn): Increased feeding to build fat reserves
- Hibernation phase (winter months): Reduced metabolic rate and activity
- Emergence phase (spring): Return to normal activity levels
This cycle repeats annually and is triggered by environmental cues like temperature and daylight hours.
Research evidence
McClintock and Stern (1998)
Research Study: Pheromone Influence on Menstrual Cycles
Participants: Women in different phases of their menstrual cycles
Aim: To investigate whether pheromones could influence menstrual cycle timing
Procedure: Researchers collected 'odourless compounds' from women's armpits during the latter half of their menstrual cycle. These compounds were transferred to other women by having them rub pads (previously wiped across donors' armpits) above their upper lip. Compounds collected from women at the beginning of their cycle were also tested.
Findings: Women who received compounds from the latter half of donors' cycles experienced shortened menstrual cycles. Conversely, those who received compounds from the beginning of cycles had lengthened cycles.
Evaluation - Strengths:
- Provides evidence that menstrual cycles can be influenced by chemical communication through pheromones
- Controlled experimental design
Evaluation - Weaknesses:
- Limited sample size and specific population studied
- Artificial laboratory conditions may not reflect natural pheromone exposure
Reinberg (1967) case study
Case Study: Isolation and Biological Rhythms
Participant: A woman living in isolation
Procedure: The woman spent three months in a cave with only a small lamp for lighting
Findings: Her daily cycle extended to 24.9 hours, whilst her menstrual cycle shortened to 25.7 days. After leaving the cave, it took a full year for her menstrual cycle to return to its normal pattern.
Significance: This study demonstrates how external zeitgebers, particularly light, can significantly influence infradian biological rhythms.
Evaluation of infradian rhythm research
Strengths
Menstrual synchrony explanation: Pheromone research helps explain why groups of women living together (such as in residential facilities) often develop synchronised menstrual cycles. This phenomenon suggests that chemical communication between individuals can coordinate biological rhythms.
Evolutionary Perspective Menstrual synchrony may have provided evolutionary benefits by allowing pregnancies to occur around similar times, enabling shared childcare responsibilities and improved survival rates for offspring.
Limitations
Critical Research Limitations
Proximity and duration factors: Research has not clearly established how close women need to live together, or for how long, before menstrual synchronisation occurs. The extent to which pheromones can influence cycles requires further investigation.
Statistical significance concerns: Wilson (1992) challenged menstrual synchrony research by re-examining existing data. The analysis revealed that experimental evidence for synchrony was often exaggerated, with results falling within normal chance variation rather than demonstrating statistically significant effects.
Key Points to Remember:
- Infradian rhythms occur less frequently than once per day, with cycles lasting weeks, months, or seasons
- The menstrual cycle is the primary example, typically following a 28-day pattern
- Pheromones can influence menstrual timing, as demonstrated by McClintock and Stern's research
- External zeitgebers like light exposure can significantly affect these rhythms
- Research evidence supports menstrual synchrony, though some studies question the statistical significance of findings