Biological rhythms - Infradian Rhythms Flashcards
what is an infradian rhythm
Infradian rhythms are biological rhythms that last longer than 24 hours.
There are two examples of infradian rhythms:
Monthly cycles e.g. the female menstrual cycle
Yearly cycles (also known as circannual rhythms) e.g. Seasonal Affective Disorder (SAD))
The menstrual cycle is driven by the
endocrine system as hormone levels (oestrogen, progesterone) fluctuate under the control of the pituitary gland, which regulates ovulation.
However, exogenous zeitgebers, such as light and odour (pheromones), are also thought to be involved
what is the average length of a cycle/ when does ovulation occur
The average length of the cycle is 28 days
Ovulation occurs around halfway through the menstrual cycle (14 days), when oestrogen levels peak and usually lasts for 16-32 hours.
The Menstrual Cycle
The pituitary gland releases FSH and LH, which stimulate a follicle in one ovary to ripen an egg and the release of oestrogen.
Once the egg has ripened, the ruptured follicle starts to secrete progesterone, which causes the lining of the womb to prepare for a pregnancy by increasing its blood supply.
~ 2 weeks after ovulation, if there is no pregnancy, progesterone is reduced, causing the lining of the womb to shed.
McClintock and Stern (1998):
Procedure
Investigated the effects of female pheromones (an exogenous zeitgeber) on the menstrual cycle.
They collected daily samples of ‘odourless compounds’ (sweat) from the armpits of a group of 9 women (20-35 years), which was then rubbed on the upper lips of a group of 20 women.
A record was kept of the participants’ menstrual cycles.
McClintock and Stern (1998):
Findings
68% of women responded to the odourless compounds (sweat).
When women received the odourless compound (sweat) from the armpits of women in the latter half of their menstrual cycle, their menstrual cycle was shortened.
If the sweat was collected from women at the beginning of their cycle, this lengthened the cycle of those who had received the compound.
This shows that the menstrual cycle of a woman can be altered by communication via pheromones. The effects of pheromones could explain menstrual synchrony
The Pheromone Myth (article) – Key arguments against the role of pheromones in menstrual synchrony
Not only have mammalian pheromones not been found, but the idea oversimplifies the nature of chemical communication among mammals. “Pheromone” has no more scientific value in describing chemically mediated behaviours or endocrine processes than “visuomones”, “audiomones”, or “touchamones” would in describing phenomena created by non-chemical stimuli.
Not surprisingly, scientists do not agree on what defines a pheromone, and attempts by chemists to identify such putative agents have failed.
Among the most publicised claims for the existence of human pheromones is menstrual synchrony. After exposure to supposed pheromones, the menstrual cycles of close friends and room-mates are said to synchronise so at some point their periods overlap. No such pheromones have been identified, and on statistical grounds alone periods will overlap for a significant amount of time in women with slightly differing cycle lengths.
Anthropologist Clyde Wilson of the University of Missouri, Columbia, and biopsychologist Jeffrey Schank of the University of California, Davis, reported that the statistical analyses used in menstrual synchrony studies are flawed in many ways.
Neither is there any convincing biological or evolutionary basis for menstrual synchrony
What might be the evolutionary advantage of menstrual synchrony?
Menstrual synchrony may occur amongst women who spend a lot of time together as it means that there could potentially be synchronised pregnancies (given that they would be ovulating at the same time).
This means that childcare could be shared when the babies are born.
However, how close women have to live together and for how long, for menstrual synchronisation to occur, is not clear.
The extent to which pheromones can have an effect still requires research
Reinberg (1976):
documented the duration of a woman’s menstrual cycle during and after she spent 3 months in a cave, with only dim lighting.
Her sleep-wake cycle lengthened slightly and her menstrual cycle became shorter during her stay in the cave.
Her menstrual cycle shortened to 25.7 days and it took a further year for her cycle to return to normal.
what does Reinberg study suggest
This study suggests that the low levels of light in the cave affected the length of the woman’s menstrual cycle.
Therefore, infradian biological rhythms (as well as circadian) can be influenced by exogenous zeitgebers i.e. light.
what is Seasonal Affective Disorder (SAD):
individuals experience severe depression during the winter months and recover during the summer.
What causes this depression in the winter months?
The hormone melatonin is secreted when it is dark (by the pineal gland).
More darkness means more melatonin, which means less serotonin (because melatonin is produced from serotonin).
Low levels of serotonin are associated with depression.
what is phototherapy
uses very strong lights (equivalent to full daylight) in the evening and/or early morning to change levels of melatonin and serotonin.
SAD sufferers have reported that daily phototherapy has relieved them of lethargy, depression and other related symptoms.
