Cecil’s latest article, http://www.straightdope.com/columns/990924.html , says:
What an experiment like this might fail to take into account is LIGHT CUES. Remember that experiment somebody-or-other did several years ago, where he shut himself up in a cave to see what his daily rhythms would do when deprived of light cues from the sun? He discovered a daily circadian rhythm existed independently of daylight/nighttime cues with a cycle period of 25 hours. Only when exposed to a dawn-and-dusk rhythm from the outside world does the human body sync up to a 24-hour daily cycle.
SO … it’s conceivable that the same thing could be happening in the human female menstrual cycle. Back in the Good Old Days before we humans invented campfires, the only significant light source at night was the moon. The human menstrual cycle may have evolved to take advantage of that, what with it being easier and safer to mate in dim light than in no light. Nowadays, though, with our nights bathed in streetlamps and house lights, there are no light-cues for the female reproductive system to synchronize with, so it would fall back to a seemingly random cycle whose period (pardon the expression) only appears coincidentally similar with the lunar light cycle.
The only way to tell for sure, of course, would be to get a bunch of women to agree to be exposed to NO light at night other than moonlight, and see what happens to their menstrual cycles. Which sounds like an expensive experiment.
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