In Shakespeare’s “Othello,” the maid Emilia tells Othello that the moon has drawn too close to the Earth — and driven men insane.

The idea that a full moon can stir up emotions, provoke bizarre behavior, and even cause physical illness isn’t just a literary trope. It’s a strongly held belief, even today.

In fact, one study says that nearly 81 percent of mental health professionals believe the full moon can make people ill.

As powerful as this ancient belief appears to be, there’s little science to back up the theory that a full moon causes a swift uptick in emergency room visits or mental health unit admissions.

Here’s what researchers have found about the moon’s effects on human bodies and behavior.

As far back as 400 B.C., physicians and philosophers blamed behavioral changes on the pull of the moon. The word “lunatic,” after all, came from the idea that changes in mental state were related to lunar cycles.

The connection between the two is even supported in historic legal treatises: Famed British jurist William Blackstone wrote that people gained and lost their ability to reason according to the moon’s shifting phases.

The possibility that humans could be affected by the moon’s cycles isn’t entirely groundless.

The ocean’s tides rise and fall in time with the moon’s phases, and several marine species — including reef coral, sea-dwelling worms, and some fish — have reproductive cycles that are roughly timed to lunar cycles.

Still, not many studies link the moon to human behavior and health conditions. Here’s what researchers can tell us about the connections that do exist between people and the full moon.

When the moon is full and bright, it may affect the quality of your sleep.

May affect sleep latency

In a 2014 analysis involving 319 people who’d been referred to a sleep center, researchers found that the full moon was associated with less deep sleep and increased REM (rapid eye movement) latency.

Sleep latency is the period between when you first fall asleep and when you enter the first stage of REM sleep. So, increased latency means it takes a longer time to get to REM sleep.

Other causes of REM sleep latency can include:

  • sleep apnea
  • alcohol use
  • some medications

Deep sleep is believed to occur during your last period of REM sleep.

May affect men and women differently

A 2015 study of 205 people found that the full moon may affect sleep differently in males and females. Many females sleep less and have less REM sleep when the full moon phase is near, whereas males have more REM sleep close to a full moon.

In 2016, a group of researchers examined the sleep cycles of children in 12 countries. They found that the children slept 1 percent less during the full moon phase. However, they didn’t find any association between this change in sleep and significant difference in behavior during that period.

Though many studies point to an association between sleep and lunar cycles, not all of them do. A 2015 study involving 2,125 people found no link between the full moon and changes in sleep patterns.

As the moon comes closer to the Earth, its gravitational pull changes — and the Earth’s large bodies of water respond with higher tides.

Scientists have wondered whether the changing gravitational pull might also affect how fluids react inside your body. Here’s what they found.

May affect blood pressure

In a 2013 study on male university students, researchers measured the effects of changing lunar cycles on the participants’ cardiovascular systems. They found that blood pressure dropped by around 5 mm Hg during new moon and full moon phases.

The students also took a step test. Their heart rate and blood pressure were both lower during full and new moons. Plus, their heart rates returned to normal levels more quickly during full and new moons.

In this study, researchers concluded that humans were more physically efficient during full and new moons. However, this finding conflicts somewhat with other research — including a 2020 study on male athletes that found no significant difference in athletic performance during different lunar phases.

The human body has adapted to eons of exposure to daylight and darkness.

This has led to the development of circadian rhythms that affect many of your body’s systems — not just your sleep-wake cycle. Circadian rhythms affect your physical and mental health, too.

But the widespread use of electric light means many of your circadian rhythms are adapting to new light and dark patterns. When circadian rhythms wobble, it can cause or increase symptoms of certain mental health disorders, including:

  • anxiety
  • bipolar disorder
  • depression
  • schizophrenia

Does the full moon still have the power to disrupt your circadian rhythm? It brightens the sky by a lowly 0.1 to 0.3 lux compared with a single streetlight (15 lux) or a cell phone screen (40 lux).

So, are full moons really associated with changes in mood and mental health?

The prevailing scientific evidence says no. Researchers in a 2017 study analyzed emergency room records at a 140-bed hospital and found that people visited the ER because of a psychiatric condition in roughly equal numbers during all four phases of the moon.

A 2006 review of more than 10,000 medical records from different facilities found the same thing: no relationship between the full moon and the number of psychiatric or mood disorder hospital visits.

Those with bipolar disorder may be affected

One marked exception to this general conclusion does exist. Researchers have found that bipolar disorder may be affected by changes in the lunar cycle. Specifically, a 2018 study examined 17 people whose bipolar disorder tended to switch rapidly from depression to mania.

The study showed that the circadian pacemaker (a small group of nerves) in these individuals became synchronized with lunar patterns. This caused changes in their sleep that then triggered a shift from depression symptoms to mania symptoms.

In a 2019 case study of a woman with bipolar II disorder, researchers suggested that this moon-mood connection can be treated by changing some medications (especially thyroid medications and antidepressants) and using light therapy.

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For decades — possibly even centuries — people have persisted in believing that there are more assaults, traumas, and suicides during full moon periods.

Numerous researchers have tackled these questions. They’ve reviewed records, conducted their own studies, and came to this conclusion: A full moon doesn’t cause an increase in these human behaviors.

In fact, two studies found that during a full moon, incidence of homicide and trauma dropped slightly.

The moon and menstrual cycles

The moon takes roughly a month to orbit the Earth. Since many menstrual cycles are about the same length, people have made a connection between the moon and its effects on menstrual periods — but they’re not actually synchronized.

A 2013 study confirms this. Over the course of a year, researchers followed 74 females through 980 periods — and they found no association between lunar phases and the onset, duration, or ending of menstrual cycles.

On the subject of human reproductive cycles, it may comfort you to know that a full moon isn’t going to send you into a sudden delivery if you’re pregnant, either.

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An illusory correlation is a kind of thinking error. It happens when you draw the wrong conclusion because you haven’t taken into account all the data — just some of it.

For example, if you visit Paris and have two unpleasant encounters with grumpy Parisians, you might come away thinking Parisians are rude. In coming to that conclusion, you’d be overlooking many positive or neutral interactions you had and focusing only on the negative ones.

Similarly, people may have noticed a behavioral episode or a traumatic incident and chalked it up to the full moon because they’ve heard myths about an association between the two.

Because the moon’s cycles are known to influence natural phenomena like the tides, some cultures have developed a persistent — but mostly incorrect— belief that lunar phases also influence human emotions, behaviors, and health.

For the most part, a full moon doesn’t cause people to become more aggressive, violent, anxious, or depressed.

There does seem to be a link between the phases of the moon and changes in symptoms of bipolar disorder. There’s also some evidence that a full moon can lead to less deep sleep and a delay in entering into REM sleep. In addition, some studies have shown a slight change in cardiovascular conditions during a full moon.

Scientists continue to study how the moon influences various physiological and psychological systems. For now, though, it appears the effect of this heavenly body on your body is less powerful than once believed.