The lunar effect is the belief that health and behavior shift with the lunar cycle, from sleep quality to mental state to physical illness. The word "lunacy" comes from luna, Latin for Moon, and "lunatic" reflected the idea that mental changes followed the Moon's phases; folklore linked the full moon to mania in bipolar disorder, seizures in epilepsy, and psychosis in schizophrenia.

The claim appears in classical Greek and Roman writing, including Aristotle and Lucilius, and was revived in the 1970s, when psychiatrist Arnold Lieber proposed that the Moon influenced the body's "biological tides" and drove violence and homicide. That theory, not a series of measurements, is the modern seed of the popular claim.

The belief persists in informal settings: police officers, hospital staff, and emergency workers often report busier nights under a full moon.

Controlled evidence does not support those claims. Literature reviews and meta-analyses find no consistent link between lunar phases and the broad behavioral and mental-health outcomes most often blamed on the full Moon. Most studies do not show a strong link between moon phases and emergency room visits, violent crime, or suicide rates, and there is little science behind the theory that a full moon causes a swift uptick in ER visits or mental health unit admissions.

A few individual studies have examined lunar phases and psychiatric hospital admissions, with one reporting a significant relationship, but isolated findings do not overturn the broader reviews.

Scale explains the gap. The Moon's gravity measurably drives the ocean's tides, because water moves freely across an entire basin. The Moon's gravitational pull on a human body is minuscule compared to its pull on the ocean, and a comparable detectable effect on human biology has not been demonstrated. Lieber's "biological tides" never acquired supporting evidence.

Sleep is the one area where studies find a modest lunar signal. In the days before a full moon, when evening moonlight is brightest, people sleep about 20 minutes less, take longer to fall asleep, and spend less time in deep, restorative sleep. Most lose only 15 to 30 minutes.

Large population studies confirm the pattern across cultures, with people tending to go to bed later and sleep less in the nights before a full moon. A 2021 study found that participants fell asleep later and slept less up to five nights before a full moon, and the effect was stronger in areas with less access to artificial light.

The likely mechanism is light. Bright evening moonlight can delay the internal clock, reduce melatonin, the hormone that signals bedtime, and keep the brain more alert. Effects are strongest in rural areas or while camping. Because the signal shows up in the nights leading to the full moon and not at random, a light cue fits the data better than the Moon acting on the body directly. Some research suggests sex differences: men appear to lose more sleep during the waxing phase, while women experience slightly less deep sleep around the full moon.

Sleep loss itself is a powerful driver of mental health problems: a single rough night can heighten anxiety and drag down mood, and ongoing sleep disruption raises the risk of depression, suicidal thoughts, and flare-ups of bipolar disorder and schizophrenia. That makes the modest sleep loss around a full moon more consequential for people already at risk, but it is not evidence that lunar phases directly cause psychiatric crises. The broader reviews find no such link.

Confirmation bias explains much of the persistence. People notice unusual events under a full moon and overlook the many unusual events on other nights, so the Moon gets credit the calendar does not support.

Illusory correlation and the availability heuristic do the rest: a busy night in an emergency room during a full moon is remembered, while a similarly busy night on an ordinary date is not.

One neurologist suggests the ancient myth about moonlight and madness may trace back to something more ordinary: restless, moonlit sleep.