Infradian rhythms are biological rhythms for which the cycle time is greater than one day. One of the clearest examples of an infradian rhythm is the menstrual cycle in women. (In other mammals the cycle is longer or shorter than the human 28 days, but is still infradian.) This cycle is governed by hormones, an endogenous mechanism. Hormones are biochemical substances that are produced by endocrine glands and released into the bloodstream. Small amounts of such hormones have a large effect on target organs such as, in the case of the menstrual cycle, the ovaries and womb. The menstrual cycle is governed by the female hormones oestrogen and progesterone. These hormones cause the lining of the womb to become engorged with blood and one egg to ripen and be released. If the egg is not fertilised, then the lining of the womb is shed. On average this cycle takes 28 days but there are large individual variations, from 20 to 60 days.

Pamela Regan (1996) is just one researcher who has demonstrated that women’s sexual desires are regulated by the ebb and flow of oestregen (increase) and progesterone (decrease) - though the cyclical nature of women’s sexual desires has been acknowledged scientifically since the time of Marie Carmichael Stopes (1918).

This endogenous rhythm can be affected by many external cues. Alain Reinberg (1967) documented the duration of a woman's menstrual cycle during and after she spent 3 months in a cave, with only dim lighting. Her sleep-wake cycle lengthened slightly and her menstrual cycle became shorter during her stay in the cave. It took a further year for her cycle to return to normal. This suggests that light can influence the menstrual cycle, probably by generally affecting the action of the suprachiasmadtic neucleus (SCN) and the circadian cycle which then has implications for the infradian rhythm.

Even more interestingly there is evidence that a woman's menstrual cycle can be entrained by the menstrual cycle of other women; an example of exogenous control, albeit biological. Martha McClintock (1971) of the University of Chicago points out the common observation that women who spend time together, such as girls living in boarding schools and nuns, appear to have synchronised menstrual cycles. McClintock proposed this effect is most likely due to pheromones, biochemical substances that act like hormones but are released into the air rather than the bloodstream. These pheromones have no smell and are not consciously detectable but they carry messages from one individual to another of the same species. Ants produce pheromones as a means of ‘telling’ other ants where to find food. Most mammals use pheromones in sexual attraction, signalling when a female is in her fertile phase. (See What is Love?) In fact the pheromones of some animals are used to produce perfume.

Support for the idea of other women’s pheromones influencing the menstrual cycle comes from a study by M J Russell, G M Switz & K Thompson (1980). Russell et al arranged to apply the pheromones of one woman to a group of sexually inactive women. The donor's pheromone substance was collected from pads placed under her arms. Once every 24 hours the pads were replaced. The old pad was then dissolved in alcohol to remove any bacteria. Finally, the pad was rubbed on the upper lip of each participant. This was repeated daily for 5 months. Some of the women were in a control group ­where they received the same treatment but did not receive the pheromone substance. Participants did not know which group they were in. A record was kept of the participants' menstrual cycles.At the end of the experiment 4 out of the 5 women in the pheromone group had menstrual cycles that synchronised to within a day of the substance donor.

But why would this happen? One possibility is that there might be an evolutionary advantage for a social group to have synchronised pregnancies so that all the women in the group were breastfeeding at the same time. This would mean that child care could be shared, which would be especially helpful if one mother died (E Bentley, 2000). Members of other animal species certainly share feeding;  and wet nursing in humans shows that you can continue to breast feed - even if your child was born much earlier.

A 1998 study by Kathleen Stern & Martha McClintock exposed a group of women to a whiff of perspiration from other women. It caused their menstrual cycles to speed up or slow down depending on the time in the month the sweat was collected - before, during or after ovulation. This led to the proposition that 2 types of pheromone influence the menstrual cycle - one, produced prior to ovulation, shortens the ovarian cycle; and the second, produced just at ovulation, lengthens the cycle.

Another example of the effect of external cues has been found in women who work with men. It has been found that they have much shorter menstrual cycles. McClintock (1971) has suggested that male pheromones may reset a woman’s biological clock. One possible explanation is that this would have adaptive value. Women who ovulate more often when men are present are likely to have more offspring and this would increase the genetic strain that has this characteristic response ­to male presence. At other times a short menstrual cycle would be a disadvantage because of the drain on resources.