Infradian Rhythms repeat themselves over a greater period of time – rhythms that last longer than a day. For example, they may repeat themselves weekly, monthly or even annually (known as a circannual rhythm), for example, hibernation, bird migration and many reproductive cycles. As an example of a human biological Infradian rhythm, take the menstrual cycle. Several research studies have been done into Infradian rhythms, and how to regulate them. As we know, they are governed by endogenous pacemakers (internal cues) and exogenous zeitgebers (external cues).
One zeitgeber that helps regulate Infradian rhythms is the release of pheromones (biochemical substances produced in the endocrine system and distributed throughout the blood, which is then released into the air. It often affects other individuals). Russell (1980) used pheromones to synchronise women’s periods within a small group. He took the pheromones of one woman and applied them to a group of sexually inactive women by taking cotton pads that had been in the woman’s armpit (on a daily basis), dissolving them in alcohol to remove any bacteria, and rubbing the cotton pads onto the group of women’s upper lips.
The same procedure was done on a control group, but the cotton pads were without pheromones. The participants didn’t know whether they were in the experimental or the control group, to reduce experimenter effects. By the end of the experiment, 4 out of 5 women in the experimental group had menstrual cycles that synchronised within 24hours of the donor’s. This shows that when several women live together (and do not take oral contraceptives), they tend to menstruate at the same time every month. However, this study lasted only 5 months.
What about the long term effects of pheromones on Infradian rhythms? McClintock & Stern (1998) set up a 10-year longitudinal follow-up study of Infradian rhythms, following Russell’s research. It involved 29 women ages between 20 and 35 with a history of irregular periods. Samples of pheromones were gathered from 9 of the women in different stages of their menstrual cycles, using the same ideas as Russell – a cotton pad in the armpit. The pads were worn for at least 8 hours to ensure that pheromones were picked up.
Again, the pads were treated with alcohol and they were frozen, to be rubbed on the upper lip of the 20 other participants. In this 10 year study, McClintock and Stern found that 68% of women responded to the pheromones and their menstrual cycles were either shortened from one to 14 days or lengthened from one to 12 days, depending on when in the cycle the samples were collected. Samples that were taken early sped up the pre-ovulatory surge of luteinising hormone, and samples that were taken later on in the cycle did the opposite, and lengthened the cycles by delaying the surge of the luteinising hormone.
Again, this shows that women who live together are more likely to menstruate around the same time every month. McClintock & Stern, however, also admitted that they were unsure as to how the pheromones actually trigger the menstrual-cycle changes – as it was rubbed on their upper lips, it could be through the skin, from inhalation, through mucus membranes, or through tiny pits in the nose, so we know that pheromones do work, but we still aren’t sure about how.
So what is menstruation like when there are no zeitgebers to work from? Reinberg (1967) studied a woman who spent three months in a cave relying on only the dim light of a miner’s lamp. Her day lengthened to 24. 6 hours but her menstrual cycle shortened to 25. 7 days. Back in the regular world following this study, it was a year before her menstrual cycle returned to normal. According to Reinberg, the level of light in the cave was what had influenced the menstrual cycle.
He also found that with 600 girls from northern Germany, the onset of menstruation which occurs at puberty (“menarche”) was much more likely to occur during the winter months – this suggests also the link between light, the secretion of melatonin and the menstrual cycle. Menarche is also recorded to be reached earlier by blind girls than by sighted girls, which suggests that the pineal gland may be affected by the secretion of melatonin – and that this affects both the menstrual cycle and (as there are more conceptions during the lighter months of the year) the reproductive cycle.
Another Infradian rhythm linked to light and melatonin that affects some people is the Seasonal affective disorder. This disorder has a circannual rhythm to it, in which people suffer from depression depending on the season, or the time of year, becoming depressed in the colder, darker winter months and recovering during the warmer, lighter summer months. Research studies show that melatonin is secreted by the pineal gland when it is dark and that there’s a positive correlation between the amount of darkness and the amount of melatonin that is secreted.
Studies have also shown that increased amounts of melatonin may be related to chronic depression. Campbell & Murphy (1998) demonstrated the importance of light as a zeitgeber by shining light on the back of the knees (as the light would not reach the eyes and that blood vessels here are very near the surface). The participants’ circadian (daily) rhythms fluctuated by as much as 3 hours. These results suggest that humans do not necessarily only rely on light that enters the eyes, and that blood may also be a messenger that carries light signals to the brain.
However, this study has never been repeated, as it is so difficult to, and it is said that participants were exposed to a small amount of light that reached their eyes, so the experiment may lack validity. There are also monthly rhythms that effect males. Empson (1977) studied 21 male participants – their body temperatures and their moods were recorded over periods varying from 49-102 days. The study found evidence that there are variations of body temperature and their own ratings of “morning alertness”, with a peak at around every 20 days.
However, results such as these are difficult to generalise, for example, the Inuit Eskimos and their Infradian rhythms. Seeing as during the winter, there is no sunlight at all, and while this is going on, menstruation stops, we can see a link between sunlight and melatonin: when it is permanently dark, melatonin levels are permanently high and this has an effect on the menstrual cycle, causing it to stop until those levels drop again.
As most of the studies are done in laboratories, we can say that most of the factors were very controlled and that any extraneous factors were non-existent – however this also means that the experiments were very unlike real life (for example, rubbing pheromones on one’s upper lip) and therefore it is difficult to generalise these results to real life – the results may not have shown such a strong result if it were done, instead, as a field study.