Abstract— A theoretical rationale is presented to justify a study of the effect of light on the variability of the menstrual cycle. The experiment consisted of exposure to light all night for several consecutive nights of cycle days 14 through 17, day one being determined by the first day of menses. Our conclusion is that we have statistical evidence that this regimen of photic, stimulation can regularize the length of the menstrual cycle and can therefore presumably influence the time of ovulation. Further studies involving much larger samples and more sophisticated measurements are fully justified at this time.