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.
Ultradian Rhythms
Ultradian rhythms are biological rhythms that occur more than once every 24 hours. The stages of sleep are a well researched example. They are controlled endogenously and are measured on an electroencephalograph (EEG). In stage one, theta waves of low amplitude and high frequency can be observed. Heart rate, breathing and temperature (HR, B&T) begin to drop. In stage two sleep spindles and k-complexes (mental responses to external stimuli) are seen. Stage three is the first stage of slow wave sleep (SWS). Here, delta waves of high amplitude and low frequency can be observed. In stage four, delta waves make up around 50% of waves and HR, B&T are at their lowest. After stage four, the sleeper ascends the ‘sleep staircase’, but instead of entering stage one they enter Rapid Eye Movement (REM) or ‘paradoxical’ sleep. HR B&T are almost as high as waking levels, the waves are theta waves and most dreaming occurs. We go through this cycle approximately 8 times in 5 hours (i.e. every 90 minutes) and there is evidence that this endogenously controlled ultradian rhythm can be influenced by external zeitgebers.
One strength of most explanations of ultradian rhythms is that they suggest an interaction between biology (endogenous pacemakers) and the environment (exogenous pacemakers). For example, they acknowledge that stage two sleep is influenced by the environment in the form of k-complexes (mental responses to external stimuli). This is positive as most psychologists a combination of external and internal factors is sensible. The explanations can therefore be praised for taking a more holistic approach.
One issue with many explanations of ultradian rhythms is that they are nomothetic. This means they assume that everyone experiences sleep in the same way. For example, in Southern Europe and South America people take ‘siestas’ in the afternoon, so their sleep pattern may be different due to altered sleeping hours. Therefore a more idographic approach (focused on the individual) would be more suitable so by taking a nomothetic approach explanations may be too simplistic.
Evidence supporting the idea that exogenous zeitgebers can affect the endogenously controlled ultradian rhythm of sleep stages comes from Dement and Wolpert. In a sleep laboratory, they sprayed REM sleepers in the face with water. They found that those sprayed reported dreaming of water more frequently than those who were not sprayed. This is positive because it seems water (an exogenous zeitgebers) can influence the ultradian rhythm of sleep stages.
One problem with this research is that it was conducted in the artificial setting of a laboratory. This means that the setting did not reflect the participants’ natural environment. For example, sleeping in a laboratory may have caused the participants to have more disturbed sleep. This is a problem as this may have caused the results to be misleading, meaning the results may have lacked internal validity.
