The most obvious example of an approximately monthly biological cycle is the human menstrual cycle. My questions are the following:

  • Is it known when and where this cycle or one like it arose?

  • What exogenous cue(s) (if any) is/are this cycle based on? The moon's orbit around the earth is seemingly obvious however the only cue that I can think of that would be easily sensed by organisms is variance in night-time illumination, which seems like a very weak phenomenon when compared to day/night and seasonal cycles. King and neap tides may be a suitable cue however they have a half-monthly modulation.

  • The moon has been steadily retreating from the earth over the course of its history, which means that months have been slowly getting longer. Is there any fossil (e.g. biogeochemical) evidence of organisms with biological cycles in synchrony with a 'short month'?

  • 2
    $\begingroup$ I don't know whether the length of the Moon's orbit is such a good explanation: the menstrual/estral cycle varies a lot amongst species. For instance, mice and rats have a 4/6 days estral cycle, chimpanzees have a 37 days menstrual cycle. Also, many animals go in anestrus (they don't cycle during certain periods of the year). $\endgroup$
    – nico
    Feb 24, 2012 at 17:19
  • $\begingroup$ @nico - Ah, that shows what I know about estrus: I was under the impression that the estral cycle was a single seasonally-modulated cycle rather than a 'cycle within a cycle'. Also didn't know that the chimpanzee menstrual cycle was different. I suspected the periodicity of coral spawning might be an example of an evolutionarily early 28-day clock but wiki seems to suggest that that's a case of an exogenous trigger rather than entrainment. $\endgroup$ Feb 27, 2012 at 3:17
  • $\begingroup$ the difference between estrous cycle and menstrual cycle is essentially that in the estrous cycle there is reabsorption of the endometrium if the animal is not fertilized. The wikipedia article explains it quite well. $\endgroup$
    – nico
    Feb 27, 2012 at 6:51

1 Answer 1


A double-blind, prospective study during the fall of 1979 investigated the association between the menstrual cycles of 305 Brooklyn College undergraduates and their associates and the lunar cycles.

.... Approximately 1/3 of the subjects had lunar period cycles, i.e. a mean cycle length of 29.5 ± 1 day. Almost 2/3 of the subjects started their October cycle in the light 1/2 of the lunar cycle, significantly more than would be expected by random distribution. The author concludes that there is a lunar influence on ovulation.

(Menstrual and Lunar Cycles, Friedmann E., American Journal of Obstetrics and Gynecology, 1981)

Another source supports this conclusion, finding that "a large proportion of menstruations occurred around the new moon."

Somewhat related, this study found that light exposure shortened menstruation cycles.

In summary, there seems to be a good amount of data suggesting that lunar cycles do in fact calibrate the length of human menstrual cycles to some degree.

  • $\begingroup$ There seems to be some data suggesting that the two are correlated. As to causation I think we're not even close to prove that. $\endgroup$
    – nico
    May 16, 2012 at 6:41
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    $\begingroup$ @nico Well, if they are correlated but lunar cycles do not affect menstrual cycles, then there are two other possibilities. Either menstrual cycles affect lunar cycles, or a confounding factor affects both lunar cycles and menstrual cycles. Neither of these seem very likely. Additionally, we know that light exposure seems to have some effect on cycle length. $\endgroup$ May 16, 2012 at 13:23
  • $\begingroup$ then why would it only work for humans and not for other primates? $\endgroup$
    – nico
    May 16, 2012 at 15:10
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    $\begingroup$ @MatthewPiziak: correct me if I'm wrong, but isn't there a third possibility as well - that the lunar cycle affects something which is the causation of the human menstrual cycle. nico - I have no idea about the cycles of other relative close ancestors to us (ie primates), but do you know if they deviate substantially from the human cycle? $\endgroup$
    – Zewz
    Dec 9, 2012 at 20:19

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