Infraduan and ultradian rhythms Flashcards
Infradian- menstrual cycle
The menstrual cycle, an example of an infradian rhythm, is governed by monthly changes in hormone levels which regulate ovulation. The cycle refers to the time between the first day of a woman’s period, when the womb lining is shed, to the day before her next period. The typical cycle takes approximately 28 days to complete (though anywhere between 24 and 35 days is generally considered normal). During each cycle, rising levels of the hormone ostrogen cause the ovary to develop an egg and release it (ovulation). After ovulation, the hormone progesterone helps the womb lining to grow thicker, readying the womb for pregnancy. If pregnancy does not occur, the egg is absorbed into the body, the womb lining comes away and leaves the body (the menstrual flow).
synchronising the menstrual cycle
Although the menstrual cycle is an endogenous system, evidence suggests that it may be influenced by exogenous factors, such as the cycles of other women. A study by Kathleen Stern and Martha McClintock (1998) demonstrated how menstrual cycles may synchronise as a result of the influence of pheromones.
Stern and McClintock studied 29 women with a history of irregular periods. Samples of
pheromones were gathered from nine of the women at different stages of their menstrual cycles, via a cotton pad placed in their armpit. The pads were worn for at least 8 hours to ensure that pheromones were picked up. The pads were treated with alcohol and frozen, to be rubbed on the upper lip of the other participants. On day one, pads from the start of the menstrual cycle were applied to all 20 women, on day two they were all given a pad from the second day of the cycle, and so on. Stern and McClintock found that 68% of women experienced changes to their cycle which brought them closer to the cycle of their ‘odour donor.
infradian rhythms-Seasonal affective disorder
Seasonal affective disorder (SAD) is a depressive disorder which has a seasonal pattern of onset, and is described and diagnosed as a mental disorder in DSM-5. As with other forms of depression, the main symptoms of SAD are persistent low mood alongside a general lack of activity and interest in life. SAD is often referred to as the winter blues as the symptoms are triggered during the winter months when the number of daylight hours becomes shorter.
SAD is a particular type of infradian rhythm called a circannual rhythm as it is subject to a yearly cycle. However, it can also be classed as a circadian rhythm as the experience of SAD may be due to the disruption of the sleep/wake cycle and this can be attributed to prolonged periods of daily darkness during winter.
Psychologists have hypothesised that the hormone melatonin is implicated in the cause of
SAD. During the night, the pineal gland (see next spread) secretes melatonin until dawn when there is an increase in light. During winter, the lack of light in the morning means this secretion process continues for longer. This is thought to have a knock-on effect on the production of serotonin in the brain - a chemical that has been linked to the onset of depressive symptoms.
ultradian rhythms-sleep cycle
One of the most intensively researched ultradian rhythms is the stages of sleep - the sleep cycle. Psychologists have identified five distinct stages of sleep that altogether span approximately 90 minutes - a cycle that continues throughout the course of the night. Each of these stages is characterised by a different level of brainwave activity which can be monitored using an EEG (see page 44).
Stages 1 and 2 This is light sleep where a person may be easily woken. In stage 1, brain waves are high frequency and have a short amplitude. These are alpha waves (see diagram on left). In stage 2, the alpha waves continue but there are occasional random changes in pattern called sleep spindles.
Stages 3 and 4 This is known as deep sleep or slow wave sleep (SWS). The brain waves are delta waves with lower frequency and higher amplitude. It is difficult to wake someone at this point.
Stage 5 (REM sleep) The body is paralysed yet brain activity closely resembles that of the awake brain. During this time, the brain produces theta waves and the eyes occasionally move around, thus rapid eye movement (REM). Dreams are most often experienced during REM sleep, but may also occur in deep sleep.
strength-evolutionary basis
One strength of menstrual synchrony research is that it may be explained by natural selection.
Synchronisation of the menstrual cycle, of the kind observed in the Stern and McClintock study (facing page), is thought by some to have evolutionary value. For our distant ancestors it may have been advantageous for women to menstruate together and become pregnant at the same time. In a social group, this would allow babies who had lost their mothers during or after childbirth to have access to breast milk, thereby improving their chances of survival.
This suggests that synchronisation is an adaptive strategy.
limitation-methodological limitations
One limitation of synchronisation studies is their methodological shortcomings.
There are many factors that may effect change to the menstrual cycle, including stress, changes in diet, exercise, etc. These may act as confounding variables, which means that any supposed pattern of synchronisation is no more than would have been expected to occur by chance. This may explain why other studies (e.g. Trevathan et al. 1993) have failed to replicate the findings.
This suggests that menstrual synchrony studies are flawed.
strength-improved understanding
One strength of research into ultradian rhythms is that it has improved understanding of age-related changes in sleep.
Sleep scientists have observed that SWS reduces with age. Growth hormone is mostly produced during SWS therefore this is reduced in older people. According to Eve van Cauter et al. (2000), the resulting sleep deficit may explain various issues in old age, such as reduced alertness. In order to increase SWS, relaxation and medication may be used This suggests that knowledge of ultradian rhythms has practical value.
limitation-individual differences
One limitation of ultradian rhythms research is there is significant variation between people.
Adrienne Tucker et al. (2007) found large differences between participants in terms of the duration of each sleep stage, particularly stages 3 and 4. Tucker et al. suggest that these differences are likely to be biologically determined
This makes it difficult to describe ‘normal sleep’ in any meaningful way.
