Welcome to Infradian Rhythms!

In the previous chapter, we explored circadian rhythms (like the 24-hour sleep-wake cycle). Now, we are going to look at biological rhythms that take longer than 24 hours to complete one full cycle. These are called infradian rhythms. We will focus on two main examples: the menstrual cycle and Seasonal Affective Disorder (SAD), as well as how light therapy can help. Don't worry if the biological terms feel new—we will break them down step-by-step!

1. What is an Infradian Rhythm?

The word "infradian" comes from Latin: infra means "below" and dies means "day." In psychology, this refers to a rhythm that happens less than once every 24 hours. In other words, one complete cycle lasts longer than a day.

  • The Menstrual Cycle: Usually lasts about 28 days (though it varies between 24 and 35 days).
  • Seasonal Affective Disorder (SAD): A yearly cycle related to the seasons.

2. The Menstrual Cycle

The menstrual cycle is a classic example of an infradian rhythm. It is regulated by internal pacemakers (our internal biological clocks) but can also be influenced by external zeitgebers (factors from the environment).

How it works:

The cycle is driven by changes in hormones like estrogen and progesterone.
1. Rising levels of estrogen cause the ovary to develop an egg and release it (ovulation).
2. After ovulation, progesterone helps the womb lining thicken to prepare for a possible pregnancy.
3. If pregnancy doesn't occur, the lining is shed (the period).

The Role of External Factors (Zeitgebers):

Although the cycle is mostly controlled internally, research suggests that external factors can influence it. For example, research into pheromones (chemical substances released into the air) has shown that when women live closely together, their menstrual cycles may synchronize. This suggests that the environment can "reset" our internal infradian clocks.

Quick Tip: Think of internal pacemakers as the "batteries" inside a watch and external zeitgebers as someone manually changing the time on the watch face!

3. Seasonal Affective Disorder (SAD)

Seasonal Affective Disorder (SAD) is a type of depression that follows a seasonal pattern. It is often called the "winter blues" because symptoms usually appear in the winter when the days are shorter and darker.

Symptoms of SAD:

  • Persistent low mood.
  • Lethargy (feeling very tired or having no energy).
  • Sleeping more than usual and craving carbohydrates.

The Biological Cause:

Psychologists believe SAD is caused by the hormone melatonin.
- The pineal gland secretes melatonin at night to make us sleepy.
- In winter, because there is less sunlight (an external zeitgeber), the pineal gland produces melatonin for longer.
- High levels of melatonin can affect levels of serotonin (a neurotransmitter linked to mood). Low serotonin is a well-known cause of depression.

4. Light Therapy

Because SAD is linked to the lack of light, one of the most common treatments is light therapy. This is a way of using external zeitgebers to fix an imbalance in our internal pacemakers.

How it works:

Patients sit in front of a very bright "light box" for about 30 to 60 minutes each morning. This light is much stronger than normal household light. It mimics sunlight, which tells the brain to reduce the production of melatonin. By lowering melatonin, serotonin levels often rise, which helps lift the patient's mood.

Did you know? Light therapy is effective for about \(80\%\) of people suffering from SAD, making it a very popular and non-invasive treatment!

5. Research and Evaluation

When studying biological psychology for your XPS01 exam, you must be able to evaluate the evidence using key terms like validity, reliability, and objectivity.

Validity (Does it measure what it claims to?)

Research into light therapy has high predictive validity because we can predict that increasing light exposure will reduce depressive symptoms. However, some critics argue that the "placebo effect" might be involved—people feel better simply because they expect the light to work, not because of the biology itself.

Reliability (Is it consistent?)

Studies into the menstrual cycle and SAD often use objective measures, such as blood tests to check hormone levels or tracking specific dates. This makes the research more reliable because other scientists can repeat the studies and get similar results.

Generalisability (Can we apply it to everyone?)

Many studies on infradian rhythms have small sample sizes. For example, some menstrual cycle studies only look at a small group of women. This means the results might not generalisable to the whole population, as every individual's biological clock is slightly different.

Objectivity vs. Subjectivity

Measuring hormone levels (like melatonin) is objective because it is based on facts and data. However, asking a patient how "depressed" they feel in winter is subjective because it relies on the person's own feelings and interpretation.

Summary Key Takeaways

  • Infradian rhythms last longer than 24 hours (e.g., menstrual cycle, SAD).
  • Internal pacemakers (hormones) and external zeitgebers (light/pheromones) work together to control these cycles.
  • SAD is caused by too much melatonin and too little serotonin due to lack of winter sunlight.
  • Light therapy uses artificial light to reset the body's clock and treat SAD symptoms.
  • When evaluating research, always consider if the data is objective (factual) or subjective (based on opinion).

Common Mistake to Avoid: Don't confuse Infradian with Circadian. Remember: Circadian = Close to a day (24 hours). Infradian = Infrequent (longer than a day).