Folklore around the world has long asserted that the moon affects human behavior and health. However, much of the research on whether the moon has any direct effect is inconclusive.

Some studies suggest the moon may have a small impact on some aspects of health, such as childbirth. But then other studies have found no connection.

Even when the moon appears to affect humans, it may not be the moon itself that is the culprit. Confirmation bias, which is the tendency to focus on information that supports what a person already believes, may help explain why people claim to observe changes in health and behavior during a full moon.

Read on to learn more about the potential effects of the full moon on energy, mood, the menstrual cycle, and more.

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It is possible that the moon could affect sleep, which may then impact peoples’ energy levels in the daytime.

Prior to artificial lighting, the full moon was a major source of nighttime light. A 2021 study states that for much of human history, people were more active at night during the full moon, and might have synchronized their activities and energy levels to the phases of the moon.

This could be because more moonlight meant people could continue working or participating in other activities later into the evening. But there is some evidence that moonlight itself may keep people awake for longer.

The study investigated the effects of the lunar cycle on people living in rural environments without electricity, in Indigenous communities, and in urban settings. By using sleep monitors, the researchers found that sleep began later and did not last as long on the nights leading up to the full moon.

This phenomenon likely occurs because the full moon is the brightest moon phase, reflecting more light from the sun to Earth. Although this light is weaker than direct sunlight, exposure may still cause people to feel more awake at night.

People anecdotally report that the full moon affects their mood, but the existing research does not support this claim.

An older 1985 meta-analysis found no connection between phases of the moon and mental health admissions to hospitals, for example. It also found that phases of the moon explained fewer than 1% of the differences in mental health admission rates.

More recently, a 2019 study of 17,966 people who sought mental health care in an inpatient setting found no connection between admissions, discharges, and the phases of the moon.

It is possible that the moon might affect mood in more subtle ways, but these are more difficult to measure, and are vulnerable to confirmation bias since people who believe that the moon affects mood are more likely to monitor and observe mood changes.

Some people believe that the moon affects the menstrual cycle. Research on this point has reached mixed or weak conclusions.

A 2021 analysis reports that, in a previous study of more than 300 females, there was a correlation between the full moon and the beginning of the menstrual period. However, this correlation only existed when researchers selected for a cycle length of 29.5 days.

To assess the connection between the moon and periods, the authors of the analysis reviewed the cycles of 22 people who kept records of their periods for up to 32 years.

They found that menstrual cycles intermittently synchronized with either the luminescence or gravitational pull of the moon. The synchronization was strongest when the moon was closest to the Earth. However, the study did not prove that the moon is the direct cause of this.

Moon phases may have an influence on childbirth. A 2021 study of a large data set that included 38.7 million births in France found small but significant variations in birth patterns related to moon phases. Births increased during the full moon, in consistency with folk beliefs.

Some other studies have found a potential link between the full moon, health, and behavior.

For example, a study from 2018 found an increase in traffic accident-related emergency transports during the full moon. The large study included 1,764 control nights, 62 full moon nights, and 842,554 emergency transports.

This does not prove the moon caused the accidents. Because there is more light during the full moon, it is possible that more people are out driving, increasing the likelihood of collisions. The light of the moon could also be a distraction to some drivers.

A 2016 study looked at prior research on crime and the phases of the moon and then separated indoor from outdoor crime. The phases of the moon did not affect indoor crime.

However, outdoor crime rates increased with the light of the moon and were higher during the full moon. Again, more light during the full moon might explain the difference.

Some research suggests that the moon might influence human health or behavior, but many of the studies on this only find a weak or small association.

It is unclear if the moon is directly responsible for these effects. The tendency to fit evidence to suit a person’s beliefs might better explain some of these connections.

Still, folk legend and some forms of spirituality claim that the moon influences behavior. It is possible that science has not captured what some societies have long believed. Without more research, scientists cannot fully discount the idea that the full moon affects health.