The lunar effect is a purported unproven correlation between specific stages of the roughly 29.5-day lunar cycle and behavior and physiological changes in living beings on Earth, including humans. In some cases the purported effect may depend on external cues, such as the amount of moonlight. In other cases, such as the approximately monthly cycle of menstruation in humans (but not other mammals), the coincidence in timing reflects no known lunar influence. A considerable number of studies have examined the effect on humans. By the late 1980s, there were at least 40 published studies on the purported lunar-lunacy connection, and at least 20 published studies on the purported lunar-birthrate connection. This has allowed several extensive literature reviews and metanalyses to be produced, which have found no correlation between the lunar cycle and human biology or behavior. The widespread and persistent beliefs about the influence of the moon may depend on illusory correlation – that is the perception of an association that does not in fact exist. As for how the belief started in the first place, a 1999 study conjectures that the alleged connection of moon to lunacy might be a ‘cultural fossil’ from a time before the advent of outdoor lighting, when the bright light of the full moon might have induced sleep deprivation in people living outside, thereby triggering erratic behaviour in predisposed people with mental conditions such as bipolar disorder. Contexts Claims of a lunar connection have appeared in the following contexts: Fertility It is widely believed that the Moon has a relationship with fertility due to the corresponding human menstrual cycle, which averages 28 days. However, no connection between lunar rhythms and menstrual onset has been conclusively shown to exist, and the similarity in length between the two cycles is most likely coincidental. Birth rate Multiple studies have found no connection between birth rate and lunar phases. A 1957 analysis of 9,551 births in Danville, Pennsylvania, found no correlation between birth rate and the phase of the Moon. Records of 11,961 live births and 8,142 natural births (not induced by drugs or cesarean section) over a 4-year period (1974–1978) at the UCLA hospital did not correlate in any way with the cycle of lunar phases. Analysis of 3,706 spontaneous births (excluding births resulting from induced labor) in 1994 showed no correlation with lunar phase. The distribution of 167,956 spontaneous vaginal deliveries, at 37 to 40 weeks