The term lunar effect refers to any real or perceived influence on humans, animals, plants, and other earthly phenomena​ that can be connected with or correlated to​ any of the Moon​'s six fundamental cycles: the five varieties of the lunar month​ (cycle averages ranging from 27.21 to 29.53 days), and the lunar day​ (24 hours 50 minutes). The lunar connection is most often made to the Moon's phases, which are a visual expression of the synodic month​. It is commonly believed that the lunar phases​ are related to unusual human behavior​​,  the onset and development of health​ issues, and the length and timing of the menstrual cycle​.[81]

The lunar effect on humans has been hotly debated by scholars for many decades. By the late 1980s, there were at least 40 published studies on the lunar-lunacy connection,[3] and at least 20 published studies on the lunar-birthrate connection.[5] To date, hundreds of studies have examined the Moon's effect on humans.[82]​​ Several extensive literature reviews and meta-analyses found no correlation between the lunar cycle and human biology or behavior.[9][73]​ According to the most extensive survey of original English-language publications on the lunar effect on humans, however, scholarly research is divided: 79 publications (45%) showed support for an effect while 98 (55%) did not. The strongest evidence for the Moon's influence was found in connection with financial markets.[82]​​[72][74]

Dr. Arnold L. Lieber, MD,[77]​ a retired psychologist and co-author of the most influential book on the topic, is in favor of the lunar effect.[75]​ Ivan W. Kelly,[76]​ a retired professor of educational psychology & special education and the subject's most prolific author, is opposed to the idea of any possible lunar influence on our lives.[73]

As described below, and in contrast to the controversial nature of the lunar effect on humans, the Moon's influence on the behaviour of animals is well established in scientific literature. 

Origins

Examples of the belief have been found in ancient Assyrian/Babylonian writing.[13] The term lunatic itself was derived from the Latin word luna, meaning "the Moon."[14]

The term lunar effect was in use by the 19th century, but this was strictly in relation to physical phenomena such as weather. In scientific literature, lunar influences are first mentioned by Charles Darwin​ in The Descent of Man​ (1871). Following his extensive studies of mammals​ and other animals, Darwin had no doubt that cycles in our lives were also subject to lunar periods.[83]​ The first modern scientific research paper discussing lunar influences on humans (published by Nobel-Prize winning Swedish scientist Svante Arrhenius​ in 1898) doesn’t use this phrase.[78]​ The first direct mention of the term lunar effect in connection with plants and animals is from 1922,[80]​ and the first use of the expression relating to humans appeared in 1957.[79]

In humans

Claims of a lunar connection to humans have appeared in many different contexts, and these are listed below by category.

Fertility

It is widely believed that the Moon has a relationship with fertility due to the corresponding human menstrual cycle, which averages 28 days.[13] Scientific opinion on the relationship between the menstrual cycle and lunar phases is divided. A comprehensive survey of research literature identified five studies that supported a lunar connection and three that did not.[82]​ 

Birth rate

Three studies carried out between 1959 and 1973 reported a 1 percent increase in births in New York following a full Moon. Notwithstanding, multiple studies have found no connection between birth rate and lunar phases. A 1957 analysis of 9,551 births in Danville, PA, 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 didn't correlate in any way with the cycle of lunar phases.[19] Analysis of 3,706 spontaneous births (excluding births resulting from induced labor) in 1994 showed no correlation with lunar phase.[21] The distribution of 167,956 spontaneous vaginal deliveries, at 37 to 40 weeks gestation, in Phoenix, AZ, between 1995 and 2000, showed no relationship with lunar phase.[23] Analysis of 564,039 births (1997 to 2001) in North Carolina showed no predictable influence of the lunar cycle on deliveries or complications.[24] Analysis of 6,725 deliveries (2000 to 2006) in Hannover revealed no significant correlation of birth rate to lunar phases.[26] A 2001 analysis of 70,000,000 birth records from the National Center for Health Statistics revealed no correlation between birth rate and lunar phase.[27] An extensive review of 21 studies from 7 different countries showed that the majority of studies reported no relationship to lunar phase, and that the positive studies were inconsistent with each other.[5] A review of 6 additional studies from 5 different countries similarly showed no evidence of relationship between birth rate and lunar phase.[28]

Blood loss

It is at times claimed that surgeons used to refuse to operate on the full Moon because of the increased risk of death of the patient through blood loss.[30] One study, in Barcelona, Spain, found a statistically significant correlation between lunar phase and hospital admissions due to gastrointestinal bleeding, but only when comparing full Moon days to all non-full Moon days lumped together.[30] The statistical significance of the results disappears if one compares day 29 of the lunar cycle (full Moon) to days 9, 12, 13, or 27 of the lunar cycle, which have an almost equal number of hospital admissions. Researchers acknowledged that the wide variation in the number of admissions throughout the lunar cycle limited the interpretation of the results.[30]

In October 2009, British politician David Tredinnick asserted that throughout a full Moon "[s]urgeons won't operate because blood clotting isn't effective and the police have to put more people on the street.".[31] A spokesman for the Royal College of Surgeons said they would "laugh their heads off" at the suggestion they couldn't operate at the full Moon.[32]

Behavior

Two studies found evidence that those with mental disorders i.e. Schizophrenia generally exhibit 1.8% of increased violent or aggressive episodes throughout the full Moon,[34][35] but a more recent study found no such correlation to that of nonschizophrenic human beings.[37] An analysis of mental-health data found a significant effect of Moon phases, but only on schizophrenic patients.[38] Such effects aren't necessarily related directly to the appearance of the Moon. A study into epilepsy found a significant negative correlation between the mean number of seizures and the phase of the Moon, but this correlation disappeared when the local clarity of the night sky was controlled for, suggesting that it was the brightness of the night that influenced the occurrence of epileptic seizures with advanced photosensitive epilepsy.[40]

A 1978 review of the literature found that lunar phases and human behaviour aren't related.[42]

Sleep

A July 2013 study carried out at the University of Basel in Switzerland suggests a correlation between the full Moon and human sleep quality.[11] Professor Cajochen and colleagues presented evidence that a lunar rhythm can modulate sleep structure in humans when measured under the highly controlled conditions of a circadian laboratory study protocol without time cues. Studying 33 volunteer subjects, the researchers found that subjective and objective measures of sleep varied according to lunar phase and thus might reflect human circalunar rhythmicity. Stringently controlled laboratory conditions, in a cross-sectional setting, were employed to exclude confounding effects such as increased light at night or the potential bias in perception. Measures of lunar influence on sleep structure, electroencephalographic activity throughout non-rapid eye movement sleep (NREM), and secretion of the hormones melatonin and cortisol, were retrospectively analyzed. At no point, throughout and after the study, were volunteers or investigators aware of the posteriori analysis relative to lunar phase. Around full Moon it was found that electroencephalogram (EEG) delta activity throughout NREM sleep, an indicator of deep sleep, decreased by 30%, time to fall asleep increased by five minutes, and EEG-assessed total sleep duration was reduced by 20 minutes. These changes were associated with a decrease in subjective sleep quality and diminished endogenous melatonin levels.[11] Cajochen said: "The lunar cycle seems to influence human sleep, even when one doesn't 'see' the Moon and isn't aware of the actual Moon phase." [51]

There are suggestions that the 2013 Cajochen study is faulty because of a relatively small sample size and inappropriate controls for gender and sex.[12] A 2014 study with a larger sample size and better experimental controls found no effect of the lunar phase on sleep quality metrics.[12] A 2016 study spanning 28 months with over 5,800 participants found a one percent alteration in sleep duration but no additional modification in activity behaviors.[52] The lead scientist said: "Our study provides compelling evidence that the moon doesn't seem to influence people's behavior."[52]

Crime

Senior police officers in Brighton, UK announced in June 2007 that they were planning to deploy more officers over the summer to counter trouble they believe is linked to the lunar cycle.[43] This followed research by the Sussex Police force that concluded there was a rise in violent crime when the Moon was full. A spokeswoman for the police force said "research carried out by us has shown a correlation between violent incidents and full moons". A police officer responsible for the research told the BBC that "From my experience of 19 years of being a police officer, undoubtedly on full moons we do seem to get people with sort of strange behaviour - more fractious, argumentative."[44]

Police in Ohio and Kentucky have blamed temporary rises in crime on the full Moon.[45][46][48] In January 2008, New Zealand's Justice Minister Annette King suggested that a spate of stabbings in the country could have been caused by the lunar cycle.[49]

A reported correlation between Moon phase and the number of homicides in Dade County was found, through later analysis, not to be supported by the data and to have been the result of inappropriate and misleading statistical procedures.[6]

Finance

On of the most promising areas of lunar effect research concerns the relationship between the Moon and financial markets​. The first modern study, examining in 2001 all major U.S. stock indexes over the prior 100 years and all major stock indexes of 24 other countries over the prior 30 years, found that returns in the 15 days around new moon dates are about double the returns in the 15 days around full moon dates.[74]​ Every research paper published since then confirmed or extended the positive connection between the Moon and the financial markets.[82]​ The most influential of these, also having the most impact on the overall lunar effect research landscape, is a 2006 publication by Kathy Yuan and others. This paper investigated the relation between lunar phases and stock market returns of 48 countries, and the findings indicated that stock returns are lower on the days around a full moon than on the days around a new moon. This lunar effect was independent of other calendar-related anomalies such as the January effect, the day-of-week effect, the calendar month effect, and the holiday effect (including lunar holidays).[72]​ 

Politics

It was suggested, by Guy Cramer, president of the aerospace science company United Dynamics Corp, that the full Moon might have influenced voter behaviour in the US 2000 Presidential Election.[50]

In animals

Correlation between hormonal changes in the testis and lunar periodicity was found in streamlined spinefoot (a type of fish), which spawns synchronously around the last Moon quarter.[59] In orange-spotted spinefoot, lunar phases affect the levels of melatonin in the blood.[59]

California Grunion fish have an unusual mating and spawning ritual throughout the spring and summer months. The egg laying takes place on four consecutive nights, beginning on the nights of the full and new Moons, when tides are highest.[16] Notwithstanding this is a well understood reproductive strategy that's more related to tides than it is to lunar phase. It happens to correlate with the lunar phase because tides are highest when the Sun, Earth, and Moon are aligned, i.e., at new Moon or full Moon.

Evidence for lunar effect in reptiles, birds and mammals is scant,[59] but among reptiles marine iguanas (which live in the Galápagos Islands) time their trips to the sea in order to reach at low tide.[60]

In insects, the lunar cycle might affect hormonal changes early in phylogenesis.[59] The body weight of honeybees peaks throughout new Moon.[59] The midge Clunio marinus has a biological clock synchronised with the moon.[11][62]

Spawning of coral Platygyra lamellina occurs at night throughout the summer on a date determined by the phase of the Moon; in the Red Sea, this is the three- to five-day period around the new Moon in July and the similar period in August.[63] Acropora coral time their simultaneous release of sperm and eggs to just one or two days a year, after sundown with a full moon.[64]

The Palolo worm that lives in the seas of Indonesia and Polynesia loose the terminal part of their bodies throughout a waning moon at a certain time of year. These parts float to the surface and release sperm and eggs. The terminal parts are gathered by people as a special food. The event would be predicted by the local priests, and the lunar calendar was set by the event. Because the Palolo adjust their spawning time between October and November, and because of inter-island differences in spawning times, there are factors additional than the Moon that control the timing. Such factor might include seawater temperatures, tides, weather, or additional biological signals.[65]

In plants

Lunar rhythms have shown to have effects on plants and agriculture. Certain moon cycles have shown positive and negative effects on the growth of plants. Six Moon configurations, namely the New Moon, the Moon opposite Saturn, the ascending and descending Moon, the Moon's nodes, perigee and apogee, and the Moon in zodiac signs have shown evidence to effect plant and vegetation growth.[53][54]

The synodic month is a cycle of 29.53 days on the average, and farmer observations and several experiments have taken place to see the effects of the familiar phases of the Moon on plant growth.[53] The Full Moon has shown to affect water and throughout this time there's quicker germination of seeds and more growth of recently mowed or cut vegetation. This occurs when the gravitational pull is the strongest throughout the new moon. The gravity causes the water to be pulled up throughout the second quarter of the new moon and water will accumulate causing the seeds to swell up. The seeds will then absorb the most water throughout the full moon.[55] When the ocean is facing the moon, the water will get pulled up towards it more than it would by the center of the earth and this process is similar to how gravity affects the rise of water in plants. Due to an increase in moisture levels, there's growth and an increase in insect activity. Faster cell division and elongation of plants additionally occurred throughout this lunar rhythm.

During the Moon's opposition to Saturn, which occurs about every 27.5 days, the Moon has a stronger attraction towards calcium.[53] The amounts of calcium stored within the plants will become more abundant and the processes of silica will additionally increase. The increase in calcium concentration in seeds will increase germination and growth rate. Calcium comes up from the root tips through apoplectic and symplastic mechanisms. Calcium is pumped through the symplast by the plasma membrane Ca2+ ATPases or Ca2+ antiporters. The calcium is delivered to the xylem.[56] Certain silica sprays have additionally shown to be effective against infectious insects and particular plant diseases.

Ascending-Descending Moon phase takes about 27.3 days to complete its cycle.[53] The ascending period consists of vast growth occurring above the soil as the nutrients will force themselves upwards. There is additionally an increase of germination occurring underneath the soil because the highest point of the lunar force occurs throughout this time.[57] The descending period of the moon can have activities that resemble a colder period for plants due to the moon force decreasing and having a lower energy impact. The forces are drawn back and most cultivation occurs throughout this time.

Meta-analyses

A meta-analysis of 37 studies that examined relationships between the Moon's four phases and human behaviour revealed no significant correlation. The authors found that, of twenty-three studies that had claimed to show correlation, nearly half contained at least one statistical error.[3][6] Similarly, in a review of twenty studies examining correlations between Moon phase and suicides, most of the twenty studies found no correlation, and the ones that did report positive results were inconsistent with each other.[6]

Proposed explanations

Believers in the lunar theory suggest several different mechanisms by which the behaviour of the Moon could influence the behaviour of human beings. A common suggestion is that, after the Moon affects large bodies of water such as the ocean (a phenomenon known as "tidal force"), the Moon should be expected to have an analogous effect on human beings, whose bodies contain a great deal of water.[13][66] Notwithstanding this is a misconception that fails to take into account differences in scale. The tidal force is in fact quite weak and should be expected to exercise no more gravitational pull on the human body than a mosquito.[13] Besides this, the "suggestion" failed to account for the dependence of tides from both the phase of the Moon and the time of day. A further suggestion is that positive ions increase in abundance throughout a full Moon and that this should be expected to influence human behavior. Notwithstanding this is a dubious claim. Not only is the increase in frequency extremely slight (much smaller than that caused by air conditioning and air pollution),[66] but ionic charge—positive or negative—has no effect on human behavior, and no physiological effect additional than static electric shock.[67]

Believers (David Tredinnick being a prominent example) often support their claims by noting that a large number of police officers, teachers, and nurses have observed a lunar effect in the course of their work. To the extent that nurses and police officers do indeed claim to observe patterns, this is most likely to be explained in terms of confirmation bias: People notice if something dramatic happens throughout a full Moon, but don't notice when nothing dramatic happens;[66] furthermore, dramatic occurrences that don't occur throughout full Moons are typically not counted as evidence against the belief.[13] Believers are further bolstered in their belief through communal reinforcement: The more people talk about the effect, the more people notice spurious relationships.[13]

Nocturnal carnivores are widely believed to have played an important role in human evolution, driving the need for nighttime shelter, the control of fire and our innate fear of darkness. We performed an extensive analysis of predatory behaviour across the lunar cycle on the largest dataset of lion attacks ever assembled and found that African lions are as sensitive to moonlight when hunting humans as when hunting herbivores and that lions are most dangerous to humans when the Moon is faint or below the horizon.

— C. Packer; A. Swanson; D. Ikanda & H. Kushnir (2011), "Fear of Darkness, the Full Moon and the Nocturnal Ecology of African Lions.", PLoS ONE 6 (7): e22285, doi: 

For some 3–4 million years, bipedal hominins in the East African Rift valley were evolving in potential conflict and competition with fearsome carnivores including sabre-toothed cats equipped with excellent night-vision. Using the largest data set ever recorded – 1,000 lion attacks on humans across Tanzania between 1988 and 2009 – Craig Packer and his colleagues showed that there's a peak of attacks by lions upon humans throughout the evening dark hours following full Moon. According to Packer, this might help explain why so a large number of myths and superstitions attribute fearsome dangers and nightmarish potencies to the Moon.[68] While not all archaeologists accept that lunar periodicity was ever relevant to human evolution, those favouring the idea include Curtis Marean, who heads excavations at the important Middle Stone Age site of Pinnacle Point, South Africa. Marean argues that anatomically modern humans around 165,000 years ago – when inland regions of the continent were dry, arid and uninhabitable – became restricted to small populations clustered around coastal refugia, reliant on marine resources including shellfish whose safe harvesting at spring low tides presupposed careful tracking of lunar phase. Against this background, if Marean is right, humans who ignored or misread the Moon might frequently have been drowned.

With gradual offshore platforms throughout spring low tides, substantial areas of the intertidal zone are revealed, and these are the most productive and safest shellfish collecting times... Foragers should schedule visits to coastal residential sites at times throughout the lunar month when spring tides are present and then move slightly inland throughout neaps to broaden the size of the exploitable terrestrial area.

— Marean, C. 2010. Pinnacle Point Cave 13B (Western Cape Province, South Africa) in context: The Cape Floral kingdom, shellfish, and modern human origins. Journal of Human Evolution 59: 425e-443 .