The idea that the phases of the moon are linked to the human psyche is one of the oldest and most pervasive examples of folk lore and mythology. It is woven into the fabric of our classic literature, poetry and music. Even today, a surprising number of people believe that our deepest emotions and mental states are influenced by the lunar cycle, and there are plenty of police officers, doctors, nurses and prison guards who would swear blind they’ve seen evidence of it in their everyday lives. But is the lunar effect real? How and why does it work? Humans have spent thousands of years discussing the lunar effect in stories and legends, and the last 40 years documenting it in the academic literature. So what’s the verdict? How does the moon affect us?
In it’s simplest form, the Werewolf exemplifies our most primitive understanding of a link between human behaviour and emotion and the moon. It captures our idea that during the full moon, man becomes wild, violent and instinctive, a reversion to a more basal, less civilised version of ourselves. This is probably the most pervasive aspect of the myth, that the moon controls human aggression, impulsivity, violence and mood. But the lunar effect has also been proposed for a range of scenarios so broad it will make your mind boggle. A quick google search will tell you that the moon controls our fertility and reproduction, influences violent crime, suicide and even traffic accidents, affects seizures, blood loss, sleep quality and even our political leanings. All of this begs the question, how and why might such a mechanism exist?
How Might the Lunar Effect Work?
Biological rhythms are common and usually controlled by hormonal changes, which are in part regulated by external stimuli such as light or temperature. In marine animals, circalunar cycles (following a lunar cycle of 29.5 days) are well documented. This is hardly surprising since the tides are heavily influenced by the lunar cycle – if you live in the water it makes sense to be in tune with it. Many marine species that sleep in the open also experience huge variation in light availability at night which may influence their biological rhythms. Other studies have found that the lunar cycle is correlated with hormonal changes in insects, fish and birds.
At a purely mechanistic level, a number of hypotheses have been proposed to explain how the moon is able to influence us. It is commonly said that water is the underlying link between physiology and the lunar cycle – the external stimulus of the lunar effect. The logic goes that if the moon exerts a tidal force on bodies of water such as seas, then it will also exert a small force on water in our bodies. This common misconception is an error of scale – the tidal force is actually extremely weak, far weaker even than the gravitational force exerted on a human by a mosquito flying close. Plus, the tidal force of the moon depends on its distance from the Earth, not its phase – the moon’s elliptical orbit takes just 27.5 days not 29.5 (the lunar cycle). Higher tides at the full and new moon occur not because tides are linked to the phases of the moon, but because at these times the Sun, Earth and Moon are aligned, increasing the tidal force. Oh yeah, and the tidal effect only works on unbounded bodies of water – the water inside your body is definitely bounded!
So, water isn’t responsible for the lunar effect. Other hypothesised triggers include electromagnetic radiation, gravitational pull, positive ions and polarisation of lunar light. There is no solid evidence for any of them, and some of the suggestions are downright stupid. What about the internal control of this hypothetical lunar effect? Several hormones have been implicated in regulating an internal lunar cycle, including melatonin, and corticosterone and other endogenous steroids. In fish, the lunar clock influences reproduction through hormonal changes initiated in the hypothalamus-pituitary-gonadal axis. Similarly, birds show daily fluctuations in melatonin and coricosterone, hormones involved in stress and sleep, which disappear during the full moon. Finally, one study has shown that the lunar cycle influences taste sensitivity in lab rats, mediated by the ultrastructure of the pineal gland. However, despite ample evidence for lunar periodicity in behaviour and physiology in animals, there is no convincing evidence for its existence in humans.
Why Might the Lunar Effect Occur?
Aside from the mechanism, what would be the purpose and benefit of evolving such a system for lunar control over the body? Biological rhythms are well documented in nature; animals, plants, even bacteria show strong daily and seasonal patterns of behaviour and physiology. It is known that these are linked both to external stimuli such as temperature and light, as well as an internal clock. These rhythms are of huge evolutionary benefit, allowing organisms to remain in sync with the day-night cycle of predator and prey activity, and annual variations in climate and food availability. Could a similar process explain the lunar effect?
One hypothesis is that if predators such as lions and sabre-toothed cats were influenced by the phases of the moon, then it would also have paid for early humans to respond to these phases. Africa lions alter their hunting behaviour in response to moonlight, and are most dangerous when the moon is faint or not visible. This is likely to be largely due to the additional light provided by the moon, making humans more vulnerable to predation during a new moon.
Another alternative explanation is that humans benefited from being in sync with the lunar phases because of their relationship with the tides and thus access to a plentiful food source – shellfish. Anatomically modern humans emerged around 165,000 years ago, a time when human populations were restricted to coastal regions by hot, arid conditions inland. These populations would have relied upon shellfish and other marine resources to survive, resources that could only be safely accessed at low tide. This could have created a strong selection pressure for humans to track and follow the lunar phases. However, this theory presupposes that lunar effects are unique to humans, perhaps an unlikely assumption. Circadian and seasonal rhythms appeared deep in our evolutionary history and are common to most life on Earth. It seems hard to believe that humans were the first life to find a benefit to a lunar rhythm, if there indeed is one.
It isn’t hard to imagine that the body could maintain a 28-day cycle of hormones and other physiological characteristics. The menstrual cycle is an example of exactly that. The question then becomes, if our behaviour, health and physiology runs on a 28-day cycle of some kind, has that evolved in relation to the lunar cycle in any way, or is it merely coincidental? The answer should be clear when we look for a real, statistical relationship between actual phases of the moon and some measure of human biology. Although the menstrual cycle is 28-days long, it does not directly relate to the lunar cycle, it simply cycles on a similar time-frame. Like two planets with the same orbit – they aren’t necessarily orbiting next to each other, but each planet completes one loop around its sun in the same amount of time. So, let’s look at the scientific evidence for a direct relationship between lunar phases and human biology.
Fertility, Reproduction and Birth
Many people claim that there is a link between fertility, birth rates, and other reproductive factors and the lunar cycle. As I already mentioned, the menstrual cycle coincidentally works to the same length cycle as the moon, which may explain the origin of these claims. Although extensively studied, there is no evidence to support a connection between specific phases of the moon and birth or reproduction. Across six separate studies, including a total of 70 million births across the US – natural, induced and cesarean section – researchers found no evidence for any relationship between birth rate and lunar phase. These findings have been corroborated elsewhere in the world; , in Germany a study of 6725 deliveries found no significant correlation between lunar phase and birth rate, across 10,027 deliveries in Ireland there was no increase in birthrate during a full moon, and a review of 21 studies covering 7 different countries showed that the majority found no relationship between lunar phase and birthrate.
Aggression, Crime and Mental Health
What about human behaviour? The lunar effect has been linked to the occurrence of homicides, suicides, fatal traffic accidents, aggravated assaults and emergency room visits. In 2007, senior police officers in Brighton announced that research had found a correlation between violent crime and the full moon , and planned to deploy more officers in response. Similarly, rises in crime have been blamed on the full moon in Ohio and Kentucky, and in New Zealand, Justice Minister Annette King suggested in 2008 that a recent spate of stabbings might relate to the full moon. As scientists rushed to study the claims empirically, some early studies found a correlation between the occurrence of abnormal behaviour and the lunar phase.
One study found a significant relationship between psychopathology and quality of life and the full moon in schizophrenic patients, however this relationship was not present for other mental health patients, and other studies have failed to find a relationship between human behaviour and the lunar cycle. For example, in Sydney, psychiatric patients show similar levels of violence and aggression throughout the lunar cycle, and a meta-analysis of 37 studies found that the lunar effect accounted for less than 1% of variation in behaviours termed ‘lunacy’. They found no effect of the lunar cycle on mental hospital admissions, psychiatric disturbances or crisis calls. A meta-analysis of 20 studies investigating the influence of the moon on suicide rates found that the majority of studies failed to find an effect; those that did show an effect were not consistent with one another, a classic sign of spurious statistics.
Further, there seems to be no discernible link between crime and the moon. Although one study in the early 1970s reported lunar periodicities in homicides, suicides, traffic accidents and assaults, closer inspection of the data revealed flawed statistical methods – the pattern did not hold up to statistical re-evaluation. Since then, an analysis of nearly 5000 crisis center telephone calls has showed no evidence for the lunar effect and less than 1% of variation in call frequency was explained by the lunar cycle. Despite this, crisis center workers reported a greater belief in lunar effects than people working in other jobs, suggesting the lunar effect may be a result of attribution and confirmation bias. Meta-analyses have also failed to find any effect of lunar cycle on homicides or other criminal offences.
Other Lunar Effects
There are some other, much stranger, things that have been linked to the lunar effect – absenteeism from work, alcoholism, electoral decisions, somnambulism (sleep walking) and even lycanthropy (werewolfism). One bizarrely common myth is that surgeons do not perform surgery on the full moon because it causes increased blood loss. Some suggest that this effect is mediated through the moon’s influence on the water in our bloodstream. However, there is no logic behind this claim, nor any robust statistical evidence to support it, and surgeons definitely do still operate, successfully, on full moon days and nights. One study investigated the influence of the full moon on nose bleeds and found there was no difference in their at the full moon, so that type of bleeding certainly seems to be unaffected. Another commonly cited effect of the lunar cycle is on seizures and epilepsy, however few studies have provided empirical evidence. One study found that nonepileptic seizures were more common during a full moon, but found no effect on epileptic seizures.
It seems wherever we look, we find a lot of anecdotal fuss about lunar effects that just doesn’t seem to hold up to empirical testing. So where does the full moon myth come from, then?
Confirmation Biases and
I touched upon an interesting point earlier – nurses, doctors, police officers and prison guards commonly report a strong belief in the lunar effect, whilst empirical studies at their places of work show it simply doesn’t exist. What can explain our stubborn insistence that we can observe an effect that can’t be measured? The explanation may well lie in psychology. Confirmation bias is a common reason why people are able to believe in patterns or relationships that aren’t real – because once an idea is planted we tend to take mental note of events that confirm our hypothesis and discard memories of those that do not. Nobody has even tried to claim that the occurrence of strange behavour or higher crime rates outside of the full moon is evidence against the lunar effect, so people tend to hold onto the idea once they’ve got it.
Where does the idea come from in the first place? The lunar effect is ingrained in classic and popular culture – it appears frequently in TV, film and books. It’s also a myth the media seems particularly fond of and it’s much harder to sell the story that the werewolf doesn’t exist. They say that one good anecdote trumps ten scientific studies when it comes to drawing and keeping the interest of the public. It simply comes down to the fact that human beings are not very good at judging chance and statistics or collecting unbiased data in the form of memories. Our brains weren’t designed for it.
It seems the lunar effect might all be in our minds, but there may be a little more to it than that…
It’s All About Sleep
To round off a sleepy October of articles (see here and here), it turns out the lunar effect might all be about sleep, as well. Some scientists, when faced with the unarguable lack of evidence for the lunar effect, have suggested that the link between behaviour, psychological wellbeing and the moon may have once existed in humans as a result of the huge influence of moonlight in a world without electric lights or candles. It’s hard to imagine just how dark a moonless night can be, and the 29.5-day cycle of light availability at night would have undoubtedly affected people’s lives and potentially their behaviour and emotions.
Furthermore, if increased light at night disturbed people’s sleep, then this may have triggered large changes in mood and behaviour during the full moon. At full moon, illuminance is 25 times greater than at the quarter moon, and 250 times greater than a moonless sky, so the lunar cycle can make a big difference to our sleeping patterns and our circadian rhythms that are regulated by light. Seizures are also more common in sleep deprived sufferers – a loss of just an hour and a half compared to normal can substantial increase the chance of a seizure the following day – which may explain the mythical link between epilepsy and the full moon. Not only that, but mania is more common in sleep-deprived bipolar patients, explaining the origin of “lunacy”.
These effects would have largely been lost, however, when humans began artificially lighting their homes, first with candles and more recently with electric lighting. When we try to look now for evidence of the lunar effect, it has evaporated as nothing more than an artefact of increased light. This idea is supported by one study which initially found a relationship between lunar phase and seizure frequency in epileptic patients, with more seizures occurring at the new moon. However, this effect disappeared when the authors statistically controlled for the brightness of the night sky, suggesting the underlying mechanism was light, not lunar.
But it might not just be about light. Research has suggested that sleep quality might vary on a lunar cycle even in the absence of the light stimulus that normal accompanies it. In 2013 a study published in Current Biology showed a correlation between sleep quality and the lunar cycle in the laboratory – even in the absence of normal light cues, sleeping people showed variation in their EEG delta activity during REM sleep in line with the lunar cycle. At the full moon, subjects took on average 5 minutes longer to fall asleep, tended to sleep for 20 minutes less and experienced less deep sleep as measured by delta activity during REM sleep. Two other studies have supported these findings, and showed that cortical reactivity, a measure of how deep you are sleeping, is higher at the full moon. People also reported feeling more tired when they awoke. Overall these findings suggest that the full moon disrupts human sleep even in the absence of light stimuli. However, a recent re-analysis of the data from these studies, in combination with fresh sleep EEG data, failed to replicate these findings. The authors also found numerous unpublished studies with conflicting results suggesting that the picture may be a lot muddier than we thought – unfortunately publication bias can make it extremely difficult to effectively evaluate the body of scientific work on a topic.
It may be too soon to make a conclusive statement on the issue of lunar influences on sleep, and in particular to what extent those influences represent an internal lunar clock rather than an extension of our circadian responses to light. However, on the whole it seems that the myth of the lunar effect is just that – folk lore from a time when the light of the moon was enough to mess with our internal body clocks, essentially giving us jet lag and exposing us to sleep deprivation that put us all in a foul mood. When we invented candles in the 19th century, we eliminated the moon’s effect on our bodies. For modern man, the moon has no discernible influence on human aggression, violence, fertility, reproduction, mental health, alcoholism, voting tendencies, traffic accidents, homicides or blood loss. What faint patterns may remain when you eliminate the stimulus of light are so minimal as to be difficult to detect statistically, which means they are probably having a pretty small effect. Still, these patterns seem likely to relate to sleep in some way, and may go so way to explaining our continued love affair with the werewolf.
Want to Know More?
- The Skeptics Dictionary (2014) Full Moon and Lunar Effects
- Cordi et al (2014) Lunar cycle effects on sleep and the file drawer problem Current Biology
- Duvdevani, Migirov, Wolf & Yakirevitch (2014) The rate of spontaneous epistaxis is not linked to the lunar cycle but shows seasonal variations Chronobiology International
- Smith, Croy & Waye (2014) Human sleep and cortical reactivity are influenced by lunar phaseCurrent Biology
- Cajochen et al (2013) Evidence that the Lunar Cycle Influences Human Sleep Current Biology
- Foster & Roenneberg (2008) Human Responses to the Geophysical Daily, Annual and Lunar Cycles Current Biology
- Roosli et al (2006) Sleepless night, the moon is bright: longitudinal study of lunar phase and sleep Journal of Sleep Research
- Zimecki (2006) The lunar cycle: effects on human and animal behavior and physiology Postepy Hig Med Dosw (Online)
- Benbadis, Chang, Hunter & Wang (2004) The influence of the full moon on seizure frequency: myth or reality? Epilepsy and Behavior
- Barr (2000) Lunacy revisited. The influence of the moon on mental health and quality of life Journal Psychosocial Nursing and Mental Health Services
- Raison, Klein & Steckler (1999) The moon and madness reconsidered Journal of Affective Disorders
- Ong (1998) Labour ward activity and the lunar cycle Journal of Obstetrics & Gynaecology
- Owen, Tarantello, Jones & Tennant (1998) Lunar cycles and violent behaviour Australian and New Zealand Journal of Psychiatry
- Gorvin & Roberts (1994) Lunar phases and psychiatric hospital admissions Psychological Reports
- Byrnes & Kelly (1992) Crisis calls and lunar cycles: a twenty-year review Psychological Reports
- Wilson II & Tobacyka (1990) Lunar Phases and Crisis Center Telephone Calls The Journal of Social Psychology
- Culver, Rotton and Kelly (1988) Geophysical variables and behavior: XLIX. Moon Mechanisms and Myths: A critical appraisal of explanations of purported lunar effects on human behavior Psychological Reports
- Durm, Terry and Hammonds (1986) Lunar phase and acting-out behaviour Psychological Reports
- Kelly, Rotton & Culver (1985) The Moon Was Full and Nothing Happened Skeptical Inquirer
- Naylor (1985) Tidally rhythmic behaviour of marine animals Symposia of the Society for Experimental Biology
- Rotton & Kelly (1985) Much ado about the full moon: A meta-analysis of lunar-lunacy research Psychological Bulletin
- Templer & Veleber (1980) The moon and madness: A comprehensive perspective Journal of Clinical Psychology
- Leiber (1978) Human aggression and the lunar synodic cycle Journal of Clinical Psychiatry
Featured image is in the public domain.