Infradian Rhythms

• What are infradian rhythms?

  A type of biological rhythm with a frequency of less than one cycle in 24 hours. eg. menstruation and seasonal affective disorder.
• Seasonal affective disorder - Variation in mood has been found; some people become more depressed during the winter months. it is governed by a yearly cycle. Hypothesis has been made that the hormone melatonin is implicated in this.
• The pineal gland secretes melatonin during the night until there is an increase of light. During the winter, melatonin is secreted longer which effects the production of serotonin - chemical linked with depressive symptoms.
• Another example of an infradian rhythm is the menstrual cycle.
• Menstrual cycle - 28-36 days. It is governed by monthly changes in hormone levels regulating ovulation. ovulation occurs halfway through the cycle when oestrogen levels are highest; usually lasting 16-32 hours. After ovulation, progestorone levels increase to prepare uterus for implantation.
• Stern and McClintock - 1998 - studied 29 women with irregular periods. a cotton pad was worn for 8 hours under the armpit to ensure pheromones were picked up. they were treated with alcohol and rubbed on the upper lip of other participants which led to 68% of women experiencing changes in their cycle donor which bought them closer to their odour donor.
• stern and mcclintock - concluded that the menstrual cycle is influenced by exogenous zeitgebers.