Infradian rhythms are biological rhythms that last for more than 24 hours. They are controlled by the hypothalamus, an endogenous pacemaker which requires input from exogenous zeitgebers so the rhythm can run correctly. An example of an infradian rhythms is the mentstrual cycle. This is controlled by the pituary gland which is an endogenous pacemaker. The pituary gland releases hormones which cause an egg in the ovaries to ripen and produce oestrogen. Whilst it is known to be biological due to its universal nature, there are large individual differences such as the length of time and the age the menstrual cycle begins.
Reinberg conducted research into infradian rhythms where a woman spent 3 months in a cave only with dim lighting. Her sleep/wake cycle slightly lengthened whereas her menstrual cycle shortened suggesting that light affects menstrual cycles. However, this is a case study which means it lacks generalisability even more so as there are large individual differences with the menstrual cycle. On the other hand, Russell et al applied female donor’s underarm sweat which has been combined with alcohol to the upper lips of female participants.
The menstrual cycles of the participants began to synchronises therefore suggesting that pheromones act as exogenous zeitgebers. Pheromones are biological substances similar to hormones but are secreted into the air and then transmitted to other animals of the same species, or in this case humans, before being absorbed into their bloodstream. However, despite the sample size being small it was a well controlled single blind study meaning that the participants didn’t know what experimental group they were in.
McClinock and Stern provided further support for the study when they found pheromones in donor’s sweat affected the recipient’s menstrual cycles, therefore suggesting that exogenous zeitgebers have a regulating effect. It may be the cases the synchronised periods have an evolutionary significance because it allows women that are living together to synchronise pregnancies so they can share child care. However, it has also been found that women working in proximity to men have shorter cycles; an evolutionary advantage which provides more opportunity for pregnancy.
Likewise, the purpose of a menstrual cycle is to conserve energy. Therefore it is better to restrict periods of fertility, highlighting the need for endogenous control. On the other hand there are ‘side effects’ of the menstrual cycle such as pre menstrual tension which has been associate with increased aggression. Lewis found that pre menstrual tensions has been cited as a defence in criminal trials. A second infradian rhythm is Seasonal Affective Disorder (SAD). It involves having severe depression in the winter months.
Levels of melatonin are higher in winter and may lead to increased depression and sleepiness. However, it has led to a treatment called phototherapy which is being exposed to bright lights for several hours a day in order to reduced the effect of too much serotonin. This in turn, lowers melatonin levels. Hamster research again has provided us with more knowledge of infradian rhythms. Male hamsters have annual rhythms of testosterone secretion based on the amount of light that occurs each day. As the breeding season begins, the days lengthen thus when it ends, they shorten.
Rusak and Zucker made lesions in the SCN, the Suprachiasmatic Nucleus, part of the hypothalamus in a hamster which destroyed their annual breeding cycles and led to male hamsters secreting testosterone all year. The 24 hour clock that daily light levels were measured against was destroyed so the seasons couldn’t be determined. Carlson found that if it was less than 12 hours of light for hamsters, it is winter where if it more than 12 hours it is summer, and therefore the breeding season. However, the research was invasive and led to brain damage, meaning it caused harm to the hamster.
As it has also been conducted on animals, it cannot be generalised because it lacks population validity despite being informative. Circa-annual rhythms which means they happen once a year are also examples of infradian rhythms. When studying migration, Gwinner kept wild birds in cages exposing them to 12 hours of light and 12 hours of darkness. This lack of external stimuli led to migratory restlessness thus suggesting there is endogenous control. However, it may be triggered by the availability of food, an exogenous cue.
As the wild birds were kept in cages, they also experienced harm because they were not in their natural setting either. Pengelly and Fisher conducted research into hibernation. Similar to Gwinner, they controlled squirrels artificially by exposing them to light 12 hours a day followed by 12 hours of darkness, keeping the temperature at 0 degrees. The squirrels hibernated from October to April. However, as the animals were not in their natural habitat, the studies lacked ecological validity because it was artificial.
The research findings are positive as they have established the existence of endogenous pacemakers regulated by exogenous zeitgebers. However, to an extent the findings are deterministic because we need exogenous zeitebers to help our endogenous pacemakers as it has been found in studies by Siffre and Aschoff and Weber that without external cues, our biological rhythms are slower than what they need to be. In addition, some of the research is idiographic because they are small samples and or case studies meaning they lack population validity.
Courtney from Study Moose
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