Does the full moon change behaviour?

Animals that keep time by the Moon

Moonlight and the tides it drives are the most reliable monthly clocks in nature. Many sea creatures in particular use them to time reproduction, so millions of individuals release eggs at the same moment.

Coral mass spawning

On Australia's Great Barrier Reef, many coral species spawn together on a few nights after the full moons of spring, usually in October, November or December. Inshore reefs tend to spawn first. The corals release bundles of eggs and sperm that float up and turn the water pink and cloudy. The event was first described in the 1980s by Peter Harrison and colleagues.

Corals have no eyes, but they carry light-sensing proteins called cryptochromes that respond to the dim blue light of the Moon (Levy and colleagues, 2007). Water temperature and day length set the season; the Moon picks the night.

Grunion: fish that come ashore

California grunion spawn on land. On spring and summer nights just after the highest tides of the month, which follow full and new moons, they ride waves up the beach. Females dig into the wet sand to lay eggs while males curl around them. The eggs incubate in the sand for about 10 days, and hatch when the next set of high spring tides washes them out to sea.

Palolo worms: a feast on the calendar

In Samoa, Fiji and other Pacific islands, palolo worms living in coral reefs release their tail sections, packed with eggs or sperm, which swarm to the surface for a night or two around the last-quarter moon in October or November. Islanders know the date in advance and scoop them up as a delicacy.

Dung beetles steer by moonlight

A dung beetle that has made a ball needs to roll it away in a straight line before rivals steal it. Marie Dacke and colleagues showed that a nocturnal African dung beetle uses the pattern of polarised moonlight to stay on course (2003). On moonless nights, beetles can even orient by the glowing band of the Milky Way (2013), the first known animal to do so.

Lions and the dark of the Moon

In the Serengeti, Craig Packer and colleagues found that lions are most successful hunters on dark nights, and that lion attacks on people in Tanzania were most frequent in the days just after the full moon, when the Moon rises well after sunset and the early evening is dark (2011).

Prey animals respond too. A study of Serengeti grazers found that animals such as wildebeest changed where and how they gathered as the moon cycle changed the risk of being caught at night (Palmer and colleagues, 2017).

Plankton under the polar night

Many tiny ocean animals (zooplankton) sink to safer, darker depths by day and rise to feed at night. During the Arctic winter, when the Sun does not rise for weeks, Kim Last and colleagues found that this daily migration switches to follow the Moon. Across the Arctic, zooplankton sank deeper when the full moon was up (2016).

Humans: the "lunar effect"

The idea is old (the word lunatic comes from luna, the Moon), and surveys find that many nurses, doctors and police officers believe it. But when researchers count carefully, the effect does not appear:

• Behaviour and mental health. Psychologists James Rotton and Ivan Kelly combined 37 studies of the Moon and behaviour in a 1985 meta-analysis, "Much ado about the full moon". They found no reliable link between the lunar phase and psychiatric admissions, crisis calls, homicides or other behaviour. Later studies, for example of psychiatric admissions and emergency evaluations at a US military hospital (McLay and colleagues, 2006), agreed.
• Births. A study of 564,039 births in North Carolina over 62 lunar cycles found no meaningful link between the moon phase and the number of births or complications (Arliss and colleagues, 2005).
• Why the myth persists. A busy night shift under a full moon is memorable ("it must be the Moon!"), while busy nights at other phases are forgotten. That is confirmation bias, and it is why counting every night matters.

Sleep: a real but small and debated effect

Sleep is the one area where serious studies have found something:

• Cajochen and colleagues (2013, Current Biology) reanalysed sleep-lab data from 33 volunteers who could not see the sky. Around full moon, they took about 5 minutes longer to fall asleep, slept about 20 minutes less and had about 30% less deep-sleep brain activity. Caveats: it was a small study analysed after the fact, and a larger reanalysis of other sleep-lab data (Cordi and colleagues, 2014) did not find the same effect.
• Casiraghi and colleagues (2021, Science Advances) used wrist activity monitors on people in indigenous Toba/Qom communities in Argentina, with and without electricity, and on 464 university students in Seattle. In all groups, people tended to fall asleep later and sleep less on the 3 to 5 nights before full moon, when the Moon is bright in the evening. Caveats: it is observational, it cannot prove the Moon is the cause, and the effect on any one night is modest.

Menstrual cycles: mostly no

The average menstrual cycle (about 29 days) is close to the lunar month (29.5 days), which has fed a long-standing belief that they are linked. Most large studies find no synchrony. Helfrich-Förster and colleagues (2021, Science Advances) analysed long-term records kept by 22 women, some for up to 32 years. They reported that cycles were intermittently in step with the Moon for some women, more often under age 35 and with less artificial light at night. Caveats: only 22 women, self-kept records, and the synchrony came and went, so it says nothing about women in general.