Infradian rhythms occur for more than one day but less than one year. The menstrual cycle is an example of infradian rhythms in humans. The human menstrual cycle in women approximately lasts 29.5 days between the ages of 15 and 40, suggested by Binkley (1997). The cycle is governed by hormones, an endogenous mechanism. The endogenous can be affected by many external cues. Reinberg (1967) documented the duration of a women’s menstrual cycle during and after she spent 3 months in a cave.
Her sleep-wake cycle lengthened slightly and her menstrual cycle became shorter during her stay in the cave. It took a further year for her cycle to return to normal. This suggests that light can influence the menstrual cycle. Furthermore, there is evidence that a women’s menstrual cycle can be entrained by the menstrual cycle of other women, an example of exogenous control. It has been a common observation that women who spend time together, such as girls living in boarding schools, appear to have synchronised menstrual cycles (McClintock (1971)).
Support for this comes from a study by Russell et al (1989). He collected daily samples of sweat from one group of women and rubbed them on the upper lips of women in a second group. Despite the fact that the women were kept physically separate, their menstrual cycles become synchronised with their odour donor. This occurred presumably because of pheromones in their sweat which affected their cycles. However, despite the implication that a pheromone is involved in the process, scientists have yet to isolate it.
A further criticism of the study is that there are a number of methodological flaws, for example the use of scientists directly involved in the research as ‘sweat donors’. Role of endogenous pacemakers and exogenous zeitgebers A biological rhythm is a biologically driven behaviour that is periodically repeated. These rhythms are governed by both endogenous and exogenous factors. Endogenous pacemakers The role of endogenous pacemakers is to set the internal body rhythms.
Most organisms have an internal body clock that matches the time passage of the 24 hour day and therefore they can control their circadian rhythm. This clock is endogenous in that it is part of their organism rather than being part of the environment. In mammals, the main pacemaker for endogenous (internal) rhythms is the suprachiasmatic nucleus (SCN). This is a small group of cells located in the area of the brain called the hypothalamus. It is called the SCN because it lies just above the optic chiasm in the brain. A neural pathway connects the retina (in the eye) to the SCN. This allows light falling on the retina to influence neurons in the SCN.
Research evidence in support for the role of the SCN as an endogenous pacemaker has been provided by Morgan (1995). Morgan removed the SCN from hamsters and found that their circadian rhythms disappeared, indicating the importance of the SCN in controlling biological rhythms. Morgan also found that if mutant hamsters are bred so they have a circadian rhythm of 20 hours instead of 24 hours and their SCN’s are then transplanted into normal hamsters, the normal hamsters will display the mutant rhythms.
This again shows the role of the SCN as a pacemaker. This is further confirmed by Silver et al (1996) who also showed that transplanted SCN’s can restore the circadian rhythm to an animal whose own SCN has been removed. The SCN appears to be the master clock, however when the SCN is removed the body temperature rhythm persists (i.e. body temperature is at its highest about half way through the waking day (early to late afternoon) and at its lowest halfway through the sleeping part of the day (3am)), which suggests that there is another clock.
For example, Folkard (1996) found that after 25 days in a cave Kate Aldcroft developed a 30 hour sleep-wake cycle but a 24 hour temperature cycle. These findings indicate that there are separate internal clocks controlling the sleep-waking cycle and temperature. Exogenous factors The role of exogenous zeitbegers is to train the body clock to external cues. As mentioned above, the SCN also receives direct information from the eye about the level of light. This means the SCN can be controlled internally but can also be reset by external cues.
In fact recent evidence shows that humans may receive information about light from elsewhere in the body. Campbell and Murphy (1998) demonstrated that participants given regular light exposure on the backs of their knees had changes in their circadian rhythms in line with the light/dark they were exposed to. However, it is not clear how this information would get to the SCN. Light is considered to be the main zeitgeber in humans. The importance of light as a time-giver can be seen in the difficulties observed in blind people.
Miles et al (1977) documented the problems of a young man who was blind from birth who had a circadian rhythm of 24.9 hours. He was exposed to various exogenous zeitgebers such as clocks and radios yet found great difficulity reducing his internal pace. This made it very difficult for him to function and as a result he had to take stimulants in the morning and sedatives at night in order to get his biological rhythm to co-ordinate his biological rhythm with the rest of the world. Therefore, this demonstrates that light really is the dominant time-giver.
However, there is also evidence that shows that where appropriate, light cues are disregarded. Luce and Segal (1966) pointed out that people who live within the Arctic Circle still sleep for about sevenhours despite the fact that during the summer months the sun never sets. In certain circumstances other external cues take over, such as social cues which dictate when it is time to get up and go to bed.
Evaluation Support for the existence of an endogenous pacemaker that is reset by exogenous zeitgebers has been provided by Siffre (1972) who was removed from the normal light-dark cycle, by being kept in a dark cave for 2 months. There were no zeitgebers such as natural light or sounds and he had no idea what time it was. He had food and drink and so on. His behaviour such as when he slept/woke and when he ate his meals was monitored. At first the findings showed there was no clear pattern in his sleep-waking cycle. However, later his sleep-waking cycle settled down to a regular pattern of about 25 hours i.e. longer the normal 24 hour cycle. This suggests that our internal biological clock must have a 25 to 30 hour cycle and that that our zeitgebers must reset the clock to our normal 24 hour day.
Therefore, in conclusion the running of the biological clock is most likely to be a combined endogenous-exogenous exercise. Both of the endogenous pacemakers and exogenous zeitgebers are important and both cause us difficulties. In terms of importance, without a biological clock an animal’s behaviour would be totally determined by environmental cues. This could be life-threatening as there would be no regularity to their behaviour. For example, Decoursey et al (2000) destroyed the SCN in some chipmunks and found that these animals were much more active at night than the normal chipmunks and were more likely to be taken by night-time predators.
On the other hand, having inbuilt biological rhythms can be problematic because they wont change when we want them to. For example, think of the difficulties people experience when working shifts. Darkness tells them they should be asleep. Also, think of the difficulties related to jet lag, the light tells us we should be wake but our internal clock says its time for sleep. Disrupting Biological Rhythms Environmental factors influencing our biological rhythms tend to change slowly, allowing the endogenous pacemakers to keep up. However, if the exogenous zeitgebers change quickly, problems can occur such as poor attention and slow reaction time. These can arise from two features of modern life; shift work and jet lag.