Biological rhythms: Infradian and ultradian rhythms.

• Infradian rhythms are a type of biological rhythms with a frequency of less than one cycle in 24 hours, such as menstruation and seasonal effective disorder.
• 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.
• 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.