infradian and ultradian rhythms - biological rhythms

• Infradian rhythms (less than one cycle in 24 hours)


  The female menstrual cycle is about 28 days. Rising of oestrogen cause the ovary to release an egg (ovulation). Then progesterone helps the womb lining to thicken, readying the womb for pregnancy. If pregnancy does not occur, the egg is absorbed and the womb lining comes away (menstrual flow).
• Infradian rhythms - Stern and McClintock

  Stern and McClintock studied 29 women with irregular periods. Pheromones were taken from some of the different stages of their cycles, via a cotton pad under their armpits. These pads were cleaned with alcohol and later rubbed on the upper lips of the other participants. 68% of women experiences changes to their cycle which brought them closer to the cycle of the odour donor.
• Infradian rhythms - SAD

  Seasonal affective disorder is a depressive disorder with a seasonal pattern. Often called the winter blues because the symptoms are triggered during the winter months when the number of daylight hours are limited and become shorter. During the night, the pineal gland releases melatonin until dawn when there is an increase in light. In winter, less light means secretion goes on for longer. This has a knock on effect on the production of serotonin in the brain.
• Ultradian rhythms (more than one cycle in 24 hours)


  Sleep patterns occur in 90 minute periods. It is divided into 5 stages which is each characterised by a different level of brainwave activity.
• Ulfradian rhythms - stages of sleep

  . Stages 1 and 2 -In stage 1, brain waves are high frequency and have short amplitude. These are alpha waves. In stage 2, alpha waves continue but there are occasional random changes in pattern called sleep spindles.

  .Stages 3 and 4 - deep sleep or slow wave sleep. The individual waves now have lower frequency and higher amplitude.

  . Stage 5 - REM (rapid eye movement) is where the body is paralysed and the brain produces theta waves and the eyes occasionally move around and this is where dreams are experienced.
• Evaluation of infradian rhythms

  One strength is its real world application. Light therapies reduces the debilitating effects of seasonal affective disorder in around 80% of people. It is also safer than using antidepressants. However, Rohan recorded a relapse rate of 46% over successive winters, compared to 27% in a comparison group receiving CBT. This suggests that light therapy may be an effective short term treatment but additional treatments may be required if benefits are to be maintained.
• Evaluation of infradian rhythms

  One strength is research on the menstrual cycle show its evolutionary basis. For our distant ancestors it may have been advantageous for females to menstruate together and become pregnant at the same time. In a social group, this would allow babies who had lost their mothers to have access to breast milk, thereby improving their chances of survival. This suggests that synchronisation is an adaptive strategy.
• Evaluation of ultradian rhythms

  One limitation is individual differences in sleep stages. Tucker found large differences between participants in the duration of stages 3 and 4 . This suggests that these differences are biologically determined. This makes it difficult to describe normal sleep in any meaningful way.