Mantegazza has suggested that the sexual period became established with relation to the lunar period because moonlight nights were favorable to courting,[80] and Nelson remarks that in his experience young and robust persons are subject to recurrent periods of wakefulness at night which they attribute to the action of the full moon.  One may perhaps refer also to the tendency of bright moonlight to stir the emotions of the young, especially at puberty, a tendency which in neurotic persons may become almost morbid.[81]

It is interesting to point out that, the farther back we are able to trace the beginnings of culture, the more important we find the part played by the moon.  Next to the alteration of day and night, the moon’s changes are the most conspicuous and startling phenomena of Nature; they first suggest a basis for reckoning time; they are of the greatest use in primitive agriculture; and everywhere the moon is held to have vast influence on the whole of organic life.  Hahn has suggested that the reason why mythological systems do not usually present the moon in the supreme position which we should expect, is that its immense importance is so ancient a fact that it tends, with mythological development, to become overlaid by other elements.[82] According to Seler, Quetzalcouatl and Tezeatlipoca, the two most considerable figures in the Mexican pantheon, are to be regarded mainly as complementary forms of the moon divinity, and the moon was the chief Mexican measurer of time.[83] Even in Babylonia, where the sun was most specially revered, at the earliest period the moon ranked higher, being gradually superseded by the worship of the sun.[84] Although such considerations as these will by no means take us as far back as the earliest appearance of menstruation, they may serve to indicate that the phases of the moon probably played a large part in the earliest evolution of man.  With that statement we must at present rest content.

It is possible that the monthly character of menstruation, while representing a general tendency of the human race, always and everywhere prevalent, may be modified in the future.  It is a noteworthy fact that among many primitive races menstruation only occurs at long intervals.  Thus among Eskimo women menstruation follows the peculiar cosmic conditions to which the people are subjected; Cook, the ethnologist of the Peary North Greenland expedition, found that menstruation only began after the age of nineteen, and that it was usually suppressed during the winter months, when there is no sun, only about one in ten women continuing to menstruate during this period.[85] It was stated by Velpeau that Lapland and Greenland women usually only menstruate every three months, or even only two or three times during the year.  On the Faroe Islands it is said that menstruation is frequently absent.  Among the Samoyeds, Mantegazza mentions that menstruation is so slight that some travelers have denied its existence.