A circadian rhythm is a biological rhythm that runs on a roughly 24 hour cycle. One example is the sleep/wake cycle. Humans sleep about 8 out of 24 hours. Even under constant light conditions, animals keep a circadian rhythm (e.g. sleeping and eating) of around 24 hours. This suggests that circadian rhythms are endogenous (internal) as they continue to run even when exogenous (external) zeitgebers are missing.
However, it seems that we rely on some exogenous zeitgebers (such as light) to entrain our circadian rhythms with a 24 hour day, otherwise they may become slightly out of sync. The main endogenous pacemaker is the suprachiasmatic nucleus (SCN), a tiny cluster of neurons located in the hypothalamus in the brain. It causes the pineal gland to secrete melatonin (a hormone that makes us sleepy) in response to low light conditions. However, even in constant light conditions the free-running SCN still controls melatonin secretion to a roughly 24 hour cycle.
One strength of explanations of circadian rhythms is that they suggest a relationship between biology and the environment. For example, they consider how the (biological) SCN works with the exogenous zeitgeber light to control the sleep/wake cycle. As most psychologists agree that both of these factors usually play a part, a combination of the two is sensible. These explanations can therefore be praised for taking a more holistic approach.
However, a weakness of explanations of circadian rhythms is that they are often nomothetic. This means that they assume circadian rhythms are the same for everyone. For example, Siffre’s research suggests a free-running sleep/wake cycle of 25 hours, but research since has shown some have a 24 hour cycle. This is problematic because it appears a more idiographic approach would be more suitable, and by being nomothetic the explanations may be too simplistic.
Evidence to support the idea of an endogenous pacemaker (the SCN) relying on an external zeitgebers (light) comes from the case study of Michel Siffre. He spent 7 months in a cave under constant artificial light conditions. His sleep/wake cycle began to free-run at about 25 hours. This suggests that we have an exogenous pacemaker that controls the sleep/wake cycle, but we rely on exogenous zeitgebers to entrain it to the 24 hour world around us, thus supporting explanations of circadian rhythms.
One drawback of this research is that it is a case study. This means that it focuses on the psychology and physiology of one person. For example, although Siffre had a free-running sleep/wake cycle of 25 hours, Aschoff and Weber’s research revealed others have them of 24 hours. This is a problem because it means that the results can’t be generalised to the entire population, giving the study low population validity.
Infradian Rhythms
Infradian rhythms are biological rhythms that occur less than every 24 hours. Examples include migration, hibernation and the human menstrual cycle. During the menstrual cycle, and egg is released approximately every 28 days through complex interactions between the brain and the reproductive organs. During each cycle, hormones such as oestrogen and progesterone are released, and physical changes happen to the breasts and reproductive organs to prepare for possible fertilisation. If the released egg is not fertilised, menstruation occurs. The cycle has also been shown to affect cognition and emotion.
Pre-menstrual syndrome (PMS) can affect many women in the week before menstruation with symptoms such as anxiety, insomnia and aggression. This is due to an imbalance of hormones. It is thought the menstrual cycle is controlled by an (internal) endogenous pacemaker, mainly the pituitary glad. However, research has shown that it can be entrained to the environment the women it in by certain external (exogenous) factors, such as pheromones.
One strength of the idea that exogenous pacemakers such as pheromones can affect infradian rhythms like the menstrual cycle is that it is supported by the evolutionary approach. This means that it has been shown to have an evolutionary advantage. For example McClinktock found women working in all male environments had shorter menstrual cycles. He concluded this gave them more opportunities to reproduce and pass on their genes. This suggests an evolutionary advantage to an endogenous infradian rhythm (menstruation) being affected by an exogenous factor (male pheromones).
However, one criticism of the idea that behaviours instigated by the infradian rhythm of PMS affect all women is that it is deterministic. This means that it suggests that all women will become ‘automatons’ of their hormones when they have PMS. For example, although some women display aggression when they experience PMS, not all women do. This is a problem because by suggesting that all women become automatons, the idea ignores the human characteristic of free will.
Evidence to support the idea that infradian rhythms can be affected by exogenous pacemakers comes from Russell et al. They rubbed pheromones from the underarms of one woman onto the top lip of five other women every day for 5 months. 4 out of the 5 women’s menstrual cycles synchronised with the pheromone donor. This suggests exogenous zeitgebers (pheromones) can affect endogenous pacemakers which run on an infradian rhythm (the menstrual cycle).
One problem with this experiment is that it relies on a small sample. This means that it attempts to generalise the results of 5 women to the entire female population. This is a problem because those 5 women may have reacted differently to the majority of women, and also because 1 of the women, representing 20% of the sample, did not conform. This is a problem because the results cannot be generalised to the rest of the population giving the experiment low external validity.