Woman, moon and menstruation.

“The pituitary has an intrinsic rhythm of small amplitude with a frequency of every two to ten minutes. Superimposed on this intrinsic rhythm is a rhythm caused by the pulsatile release of hypophysiotropic releasing factors, with or without the withdrawal of a corresponding inhibitory factor. Rhythms that are shorter than a day are referred to as ultradian rhythms. The next layer of rhythmicity is the circadian rhythm, i.e., rhythms with approximaely 24-hour period by a periodic environmental cue such as dark-light cycle. The suprachiasmatic nucleus functions as a circadian pace maker and receives light-induced electrical impulses from the retina via the retino-ophthalmic tract, finally transmitting those impulses to the pineal gland, where they are converted to hormonal signals. Signals for a rhythm with a periodicity longer than 24 hours, i.e., an infradian rhythm, include the gravitational influence of the moon, which gives rise to the menstrual cycle.”