Thanks Topsie. Got there before I did. Not only does the cycle differ from woman to woman, it also differs in the same woman at different times in her life. In the absence of males, it can be much longer than 28 days. (Research carried out with nuns, along with work on women synchronising their cycles.) At times of stress the cycle varies as well.

If it were linked to the Moon, it would presumably also be linked to the Moon in our nearest primate relatives, much more outdoors than we are, and it isn't consistently so. 29 days in orangutans - closer to the actual lunar cycle than 28. 30 in gorillas. In view of the sample size, these are not significantly different from us. But in chimps, 35 to 37 days, and bonobos 34 to 36 days. But these are averages, and individuals vary, as in humans. In bonobos, cycles of 17 and 65 days have been observed.

Penny