Specification: Biological rhythms: Infradian and ultradian and the difference between these rhythms.

Infradian rhythms

Another important biological rhythm is the infradian rhythm. Infradian rhythms last longer than 24 hours and can be weekly, monthly or annually.

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 lasts for 16-32 hours. After the ovulatory phase, 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 longer cycles of up to 36 days.

A second 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 has been linked to the depressive symptoms.

Exam Hint: While it is logical to assume that infradian rhythms, in particular the menstrual cycle, are governed by internal factors (endogenous pacemakers) such as hormonal changes, research suggests that these infradian rhythms are heavily influenced by exogenous zeitgebers.

Evaluating infradian rhythms

Research suggests that the menstrual cycle is, to some extent, governed by exogenous zeitgebers (external factors). Reinberg (1967) examined a woman who spent three months in a cave with only a small lamp to provide light. Reinberg noted that her menstrual cycle shortened from the usual 28 days to 25.7 days. This result suggests that the lack of light (an exogenous zeitgeber) in the cave affected her menstrual cycle, and therefore this demonstrates the effect of external factors on infradian rhythms.

There is further evidence to suggest that exogenous zeitgebers can affect infradian rhythms. Russell et al. (1980) found that female menstrual cycles became synchronised with other females through odour exposure. In one study, sweat samples from one group of women were rubbed onto the upper lip of another group. Despite the fact that the two groups were separate, their menstrual cycles synchronised. This suggests that the synchronisation of menstrual cycles can be affected by pheromones, which have an effect on people nearby rather than on the person producing them. These findings indicate that external factors must be taken into consideration when investigating infradian rhythms and that perhaps a more holistic approach should be taken, as opposed to a reductionist approach that considers only endogenous influences. Evolutionary psychologists claim that the synchronised menstrual cycle provides an evolutionary advantage for groups of women, as the synchronisation of pregnancies means that childcare can be shared among multiple mothers who have children at the same time.

There is research to suggest that infradian rhythms such as the menstrual cycle are also important regulators of behaviour. Penton-Volk et al. (1999) found that woman expressed a preference for feminised faces at the least fertile stage of their menstrual cycle, and for a more masculine face at their most fertile point. These findings indicate that women’s sexual behaviour is motivated by their infradian rhythms, highlighting the importance of studying infradian rhythms in relation to human behaviour.

Finally, evidence supports the role of melatonin in SAD. Terman (1988) found that the rate of SAD is more common in Northern countries where the winter nights are longer. For example, Terman found that SAD affects roughly 10% of people living in New Hampshire (a northern part of the US) and only 2% of residents in southern Florida. These results suggest that SAD is in part affected by light (exogenous zeitgeber) that results in increased levels of melatonin.

Evaluating Ultradian rhythms

Individual Differences: The problem with studying sleep cycles is the differences observed in people, which make investigating patterns difficult. Tucker et al. (2007) found significant differences between participants in terms of the duration of each stage, particularly stages 3 and 4 (just before REM sleep). This demonstrates that there may be innate individual differences in ultradian rhythms, which means that it is worth focusing on these differences during investigations into sleep cycles. o In addition, this study was carried out in a controlled lab setting, which meant that the differences in the sleep patterns could not be attributed to situational factors, but only to biological differences between participants. While this study provide convincing support for the role of innate biological factors and ultradian rhythms, psychologists should examine other situational factors that may also play a role.

Additionally, the way in which such research is conducted may tell us little about ultradian rhythms in humans. When investigating sleep patterns, participants must be subjected to a specific level of control and be attached to monitors that measure such rhythms. This may be invasive for the participant, leading them to sleep in a way that does not represent their ordinary sleep cycle. This makes investigating ultradian rhythms, such as the sleep cycle, extremely difficult as their lack of ecological validity could lead to false conclusions being drawn.

An interesting case study indicates the flexibility of ultradian rhythms. Randy Gardener remained awake for 264 hours. While he experienced numerous problems such as blurred vision and disorganised speech, he coped rather well with the massive sleep loss. After this experience, Randy slept for just 15 hours and over several nights he recovered only 25% of his lost sleep. Interestingly, he recovered 70% of Stage 4 sleep, 50% of his REM sleep, and very little of the other stages. These results highlight the large degree of flexibility in terms of the different stages within the sleep cycle and the variable nature of this ultradian rhythm.

Possible exam questions

1. Outline one or more examples of ultradian rhythms. (4 marks)

2. Outline one or more examples of infradian rhythms. (4 marks)

3. Outline what research into ultradian/infradian rhythms has found. (4 marks)

4. Outline and evaluate infradian and/or ultradian rhythms. (16 marks)