Infradian rhythm

An infradian rhythm is a biological rhythm with a period longer than 24 hours.

Menstrual Cycle
The menstrual cycle is an example of an infradian rhythm. The endogenous pacemaker of this is the actual biological cycle. Oestrogen and progesterone are released by the ovaries. At day 14, there is a peak in oestrogen when the egg is released, this is due to the release of lutenising hormone and follicle stimulating hormone from the pituitary gland. The progesterone maintains the thickness of the womb lining, and a sudden drop in this, produces the period.

The exogenous zeitgeber example of the menstrual cycle is pheromones. When several women live together, their menstrual cycles tend to syncronize due to pheromones. Pheromones are biological substances that are released into the air. It's possible this is due to social trends- women able to give birth at the same time, and so can share child rearing responsibilities.

Russell et al. applied the pheromones of one woman, to a group of sexually inactive women. A swab was taken from the donors armpits daily, and this was rubbed on to the upper lip of the women (after having removed the bacteria). It was found that 4/5 of the women syncronized with the donor woman- emphasizing that exogenous zeitgebers play a large role in the regulation of bodily functions.