Biological rhythms: Infradian and ultradian rhythms.

• Infradian rhythms are a type of biological rhythm that takes more than a day to complete
• An ultradian rhythms is a type of biological rhythm with a frequency of more than one cycle in 24 hours, such as the stages of sleep.

  
  

  Stage 1/2: Light sleep where a person can be easily woken. In stage 1, brain waves have high frequency and short amplitude (alpha waves). In stage 2, alpha waves continue with occasional random changes in pattern.

  Stages 3/4: Known as deep sleep where the waves have lower frequency and higher amplitude. Difficult to wake someone.

  Stage 5: REM sleep where the body is paralysed yet the brain activity resembles that of an awake brain. Brain produces theta waves and eyes move around (rapid eye movement). Dreams occur mostly during REM sleep.
• Menstrual cycle


  Example of an infradian rhythm, governed by monthly changes in hormone levels which regulate ovulation. The cycle refers to the time between the first day of a womens period, when the womb lining is shed, to the day before her next period.

  During each cycle, rising levels of the hormone oestrogen cause the ovary to developed an egg and release it.
• Synchronising menstrual cycle


  The menstrual cycle if an endogenous system, but evidence shows that it can be influenced by exogenous factors such as the cycles of other women.

  Stern & McClintock studied 29 women with irregular perods. Samples of pheromones were gathered from 9 women, and rubbed on the upper lips of other participants.

  ->68% of women experienced changes to their cycle.