IN FEBRUARY 1954, biologist Frank Brown discovered something that made no sense. While investigating whether oysters can keep time, he had found that they open their shells to feed at high tide, roughly twice a day. Brown had a hunch they weren鈥檛 simply responding to changes in their environment but would continue the rhythm even if moved far from the sea. To find out, he shipped a batch of oysters from the ocean off New Haven, Connecticut, hundreds of kilometres inland to Northwestern University in Evanston, Illinois.
Brown kept the shellfish in a sealed darkroom, shielded from changes in temperature, pressure, water currents and light. At first, the oysters kept their rhythm, feeding each day in time with the New Haven tides. Then, something strange happened 鈥� their feeding times gradually shifted until they lagged 3 hours behind. Brown was mystified, until he realised that they had adapted to the local state of the moon: they were . Despite having no obvious environmental cues, it seemed these shellfish were somehow tracking lunar cycles.
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Brown became convinced that oysters, humans and all life forms are plugged into subtle cosmic cues, continuously sensing both lunar and solar movements to coordinate biological processes, from metabolism to reproduction. But his ideas seemed outlandish to his peers. Brown鈥檚 results were forgotten, and the notion of lunar influences was dismissed as pseudoscience. Now, growing evidence from a range of fields suggests he might have been right. Perhaps, hidden beneath our more obvious earthly rhythms, we are also creatures of the moon.
The belief that the moon guides life on Earth goes back millennia. The ancient Greeks thought female fertility had a lunar source, noting that the menstrual cycle averages 29.5 days, the same as the period between consecutive full moons. The word 鈥渕enstruation鈥� is derived from the Greek mene, meaning moon. The moon has also traditionally been believed to influence mental health, hence the pejorative term 鈥渓unatic鈥� (from the Latin luna, or moon).
When the foundations for the modern field of chronobiology were laid in the 1950s and 60s, however, most researchers focused on daily rhythms. These appeared to be driven by internal, biochemical timers nudged by physical cues, especially sunlight.
Shadow of the sun
A key line of evidence came from a bunker built into a Bavarian hillside by German physiologist J眉rgen Aschoff. Starting in 1964, hundreds of volunteers lived alone in the bunker for months at a time, shielded from sunlight and any clues to the time of day. It had already been established that our sleep-wake rhythms, as well as aspects of our physiology ranging from body temperature to metabolite excretion, vary with the 24-hour day. In the bunker, these cycles persisted, but they lagged a bit slower than normal. Aschoff concluded that Brown was wrong: our daily rhythms don鈥檛 respond to unidentified cosmic signals, otherwise they would presumably still keep perfect time. Although usually entrained by the sun and other environmental cues, our internal clocks can carry on running independently.
Chronobiology became hugely successful. Since the 1980s, we have identified multiple genes that encode proteins that regulate each other in a complex network of feedback loops. This creates a steady cycle that pulses once each day in time with the sun. Such circadian clocks are now recognised as a defining feature of life: they tell animals when to feed, run, sleep and digest, help plants ration their starch reserves through the night and signal thousands of ocean plankton species to avoid predators by sinking before dawn and rising each evening in the largest movement of biomass on the planet.
鈥淓vidence is growing that the influence of the moon is more widespread than we thought鈥�
By tracking shifting times of sunrise and sunset, these clocks drive seasonal changes too, telling creatures when to migrate, moult and reproduce. In medicine, the implications of living out of sync with the solar day are now known to include insomnia, depression, obesity, heart disease and cancer.
Lunar influences, on the other hand, have received less attention, in part due to their colourful folk history. As chronobiology took off as a serious field of study, mentions of the moon were discredited or ignored, and researchers have conventionally investigated solar effects in isolation, as if the moon didn鈥檛 exist. Now, evidence is growing that the impact of the moon is more widespread than previously thought. We have long known that its pull on our planet causes the oceans to bulge out, leading to high and low tides in coastal areas, but how the moon influences life in the seas is also becoming clearer.
Aquatic species that reproduce by releasing eggs and sperm into the water often use lunar phases to help time their spawning to a tight window. On Australia鈥檚 Great Barrier Reef, hundreds of millions of corals release so many eggs and sperm in moon-driven mass events that they turn the ocean pink. Meanwhile, many tidal creatures depend on extra-large spring tides that happen twice each month when Earth, the moon and sun are aligned in a way that creates extra gravitational pull. Marine midges, which live in algal mats off rocky Atlantic shores, emerge as adult flies only when the seabed is exposed at the lowest low tide. Japanese land crabs anticipate high tides, climbing down from the mountains where they live to release their offspring into the sea.
Many inland animals also rely on the light of the moon to reproduce and survive. Predators including owls, wolves and snakes hunt by moonlight. North American birds called whip-poor-wills hatch during the new moon so the chicks鈥� highest energy demand comes two weeks later, when there is moonlight to help their parents catch insects. Serengeti wildebeest use the moon to time conception so calves are born safely ahead of the species鈥� spring migration. And in 2015, biologists reported the first example of lunar reproduction in a plant. Ephedra foeminea, a relative of conifers and cycads, attracts insects during the full moon by exuding drops of sugary liquid that glitter like diamonds in the moonlight.
These effects can shape entire ecosystems. In 2016, biologists reported that in the Arctic Ocean during midwinter, when the sun doesn鈥檛 rise, the mass daily sinking of oceanic plankton , following the moon.
The mechanisms behind such behaviour have remained elusive. Are organisms passively responding to environmental changes, or are they driven by internal lunar clocks? Over the past few years, genetic studies have shown that lunar cycles directly regulate biology in marine species such as coral, bristle worm and rabbitfish. A 2017 study of Acropora gemmifera coral from Taiwan found that the activity of hundreds of genes varied with lunar phases, including such as cell signalling and cell division. What鈥檚 more, several studies have now shown that these cycles continue in constant laboratory conditions.
Another surprising discovery was that many of the genes that vary with lunar phases are involved in the circadian clock. A 2019 study found that when oysters were kept in constant conditions, several genes thought of as core components of the circadian clock .
鈥淭hat would suggest that a part of the circadian clock can do a tidal oscillation as well,鈥� says , who studies molecular mechanisms of lunar cycles at the University of Vienna, Austria. The clock machinery in these marine species is more complex and thought to be evolutionarily more ancient than that of mice, humans or fruit flies. She says it suggests that, at least for short cycles on the scale of hours, life鈥檚 ancestral clock could actually be tuned to the sun and the moon.
This new evidence highlights the problem of considering solar and lunar effects in isolation. 鈥淚 think this is wrong,鈥� says Tessmar-Raible. The approach can be useful for unravelling molecular mechanisms in the lab, she argues, but doesn鈥檛 make sense from an ecological point of view. 鈥淣ature is a combination of sun and moon.鈥�
The idea that there are lunar effects on our own body clocks has been particularly contentious. Studies attempting to link events such as violent behaviour, the menstrual cycle or births to lunar phases have yielded mixed results, with rigorous, recent studies often finding no effect. Some have argued that there is no point even doing further research; the very idea of such influences was 鈥�.
But Tessmar-Raible points out that because genetically controlled lunar rhythms have now been found across the animal kingdom, from fish to invertebrates, it wouldn鈥檛 be surprising if they exist in some form in humans too. Michael Smolensky, a chronobiologist at the University of Texas at Austin, agrees. Lunar effects may have disappeared in more recent studies because we are no longer exposed to natural patterns of moonlight, he says. 鈥淗umans have been separated from their environment.鈥�
There is plenty of research keeping the debate open. A 2019 study of more than 2600 suicides in northern Finland found that the rate increased at full moon, but only in premenopausal women. The researchers suggested that this could be , as its hormonal changes correlate with feelings of depression.
German researchers reported to the European Biological Rhythms Society last year that moonlight can set the rhythm of menstrual cycles. Several recent studies have also found that sleep quality varies with the phase of the moon 鈥� with shorter, poorer sleep at new moon 鈥� even in constant lab conditions, although others haven鈥檛 seen this effect. Disrupted sleep, in turn, can trigger seizures and exacerbate conditions such as bipolar disorder and schizophrenia.
Perhaps most intriguing of all is a 2018 study by Thomas Wehr, a psychiatrist and sleep researcher at the US National Institute of Mental 午夜福利1000集合. He studied people with bipolar disorder who tracked their sleep times and mood over several years, and concluded that switches between high and low mood were triggered by varying sleep patterns linked to lunar cycles. One of the patients, who recorded his sleep times for 17 years, went to sleep at the same time each day but his waking time was on a 24.8-hour cycle, with the moon. That meant sometimes he got hardly any sleep, and other times he slept most of the day. Wehr suggests .
Philip Lewis, who studies circadian biology at the University of Cologne, Germany, describes Wehr鈥檚 work as 鈥渁bsolutely fascinating鈥�. Back in Aschoff鈥檚 bunker in the 1960s, when volunteers were shielded from the sun, the timing of their daily cycles slipped, a result that helped to kill off Brown鈥檚 idea of cosmic cues. But the average shift was to a 24.8-hour period 鈥� the length of the lunar day, Lewis points out. What if, in the absence of solar cues, these human subjects were somehow following the moon? The timing is 鈥渟uspicious鈥�, agrees Tessmar-Raible, though she emphasises there is no direct evidence as yet that this effect is caused by the moon.
鈥淕enetically controlled lunar rhythms have been found across the animal kingdom鈥�
Crucially, the effects seen in Wehr鈥檚 study don鈥檛 match up with lunar phases, so couldn鈥檛 have been caused by varying levels of moonlight. Instead, the people鈥檚 sleep seemed to be influenced by tidal cycles 鈥� but by what mechanism might the people studied by Aschoff and Wehr sense lunar movements?
One speculative suggestion is gravity. There are studies hinting that subtle gravitational changes can influence growth in plants, for example. Another possibility was first suggested by Brown. Back in the 1950s, he wanted to know how his oysters were tracking the moon. He filled mazes with thousands of more mobile invertebrates 鈥搈ud snails and flatworms 鈥� and found that they oriented according to very weak magnetic fields, with a preferred magnetic direction that changed according to the lunar phase. The movements of the sun and moon create subtle ripples in Earth鈥檚 magnetic field; Brown suggested that an ability to sense these changes explained his mysterious results.
After that, Brown was dismissed even more harshly. Earth鈥檚 magnetic field is extremely weak, about 200 times weaker than a fridge magnet, and the lunar ripples are even tinier. There was no known mechanism by which such small changes could influence biological processes. Yet even though many shunned his work in public, they didn鈥檛 ignore it completely. Aschoff, working with a physicist called R眉tger Wever, actually built two bunkers. They were identical except for one thing: one was shielded from Earth鈥檚 magnetic field. If the volunteers鈥� daily rhythms continued to run regardless, it would prove Brown wrong, or so the idea went. They did indeed continue, clinching the notion of an internal clock, but the results from the two bunkers weren鈥檛 the same.
In the shielded bunker, the subjects鈥� rhythms slipped even further behind and became more variable. Sometimes, different body clocks became uncoupled. In the 1970s, Wever published the results in a series of now-obscure papers. He described the effects as 鈥渞emarkable鈥�, But Aschoff didn鈥檛 mention the shielded bunker in his own high-profile studies, and chronobiology research continued as if the experiment never happened.
Biologists studying navigation, however, have since realised that animals can sense vanishingly weak magnetic fields through a variety of methods. Fish use networks of jelly-filled canals to measure the flow of current as they swim through a field. Some bacteria use tiny magnetic crystals to steer themselves along field lines, and similar crystals are found in lots of species, including humans. Another mechanism appears to involve light-sensing molecules called cryptochromes. Light pushes the molecule into an activated state, and sometimes even a tiny nudge from a magnetic field can influence the rate at which this occurs.
Cryptochromes have a wide range of functions, from regulating growth rate and flowering in plants to enabling magnetic navigation in birds and butterflies. Intriguingly, they are also involved in biological clocks. Neuroscientist Steven Reppert at the University of Massachusetts says that in monarch butterflies, magnetosensing for navigation seems to occur independently of a biological clock. However, magnetic fields have been shown to alter the period length of the circadian clock in several insect species including cockroaches.
There are now even hints that humans are sensitive to magnetic fields after all. In 2011, Reppert inserted a human cryptochrome into fruit flies whose own cryptochrome had been disabled and . So it is possible that cryptochromes function as magnetosensors in humans, he says, 鈥渂ut more work is needed鈥�. Meanwhile, Joseph Kirschvink at the California Institute of Technology, who believes that magnetite crystals play this role instead of cryptochromes, reported last year that
Perhaps alongside the more obvious cues such as light and temperature, subtle magnetic ripples really are keeping our bodies and brains in constant touch with the movements of the moon and sun. 鈥淧robably most people would laugh about it, but I would not keep it out of the equation,鈥� says Tessmar-Raible. 鈥淚t鈥檚 definitely something we should look into if we want to understand this.鈥� A first step, she suggests, might be to test whether the lunar cycle in model species such as bristle worms or marine midges can be shifted using magnetic cues.
Half a century ago, Brown described living creatures, including humans, as an inseparable part of an electromagnetic cosmos. Organisms and their environment, he said, 鈥溾��. His vision was discarded in favour of autonomous, biochemical clocks, tuned by sunlight. Yet for all the success of that model, maybe Brown, too, was onto something. 鈥淎nimals use all available timing cues and this includes the moon equal to the sun,鈥� says Tessmar-Raible. 鈥淣ow, the more people look, the more they find.鈥�
is a journalist based in London. Her new book聽The Human Cosmos: A Secret History of the Stars聽is out now (Canongate/拢16.99). The book is available to purchase online at聽听补苍诲听.
