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Sleep Starts Later and is Shorter on Nights before Full Moon

By Science News Staff / Source

Before the availability of artificial light, moonlight was the only source of light sufficient to stimulate nighttime activity; still, evidence for the modulation of sleep timing by lunar phases is controversial. 

A new study, led by the University of Washington, shows a clear synchronization of nocturnal sleep timing with the lunar cycle in participants living in environments that range from a rural setting with and without access to electricity in indigenous communities in northern Argentina to Seattle, a highly urbanized postindustrial setting in the United States. 

The results, published in the journal Science Advances, show that sleep starts later and is shorter on the nights before the full moon when moonlight is available during the hours following dusk.

“We see a clear lunar modulation of sleep, with sleep decreasing and a later onset of sleep in the days preceding a full moon,” said lead author Professor Horacio de la Iglesia, a researcher in the Department of Biology at the University of Washington, Seattle.

“And although the effect is more robust in communities without access to electricity, the effect is present in communities with electricity.”

Using wrist monitors, Professor de la Iglesia and colleagues tracked sleep patterns among 98 individuals living in three Toba/Qom indigenous communities in the Argentine province of Formosa.

The communities differed in their access to electricity during the study period: one rural community had no electricity access, a second rural community had only limited access to electricity, while a third community was located in an urban setting and had full access to electricity.

For nearly three-quarters of the Toba/Qom participants, researchers collected sleep data for one to two whole lunar cycles.

Toba/Qom in the urban community went to bed later and slept less than rural participants with limited or no access to electricity.

But study participants in all three communities also showed the same sleep oscillations as the moon progressed through its 29.5-day cycle.

Depending on the community, the total amount of sleep varied across the lunar cycle by an average of 46 to 58 minutes, and bedtimes seesawed by around 30 minutes.

For all three communities, on average, people had the latest bedtimes and the shortest amount of sleep in the nights three to five days leading up to a full moon.

When they discovered this pattern among the Toba/Qom participants, the researchers analyzed sleep-monitor data from 464 Seattle-area college students that had been collected for a separate study. They found the same oscillations.

“We hypothesize that the patterns we observed are an innate adaptation that allowed our ancestors to take advantage of this natural source of evening light that occurred at a specific time during the lunar cycle,” said first author Dr. Leandro Casiraghi, a postdoctoral researcher in the Department of Biology at the University of Washington, Seattle.

The team also found a second, semilunar oscillation of sleep patterns in the Toba/Qom communities, which seemed to modulate the main lunar rhythm with a 15-day cycle around the new and full moon phases.

This semilunar effect was smaller and only noticeable in the two rural communities.

“Future studies would have to confirm this semilunar effect, which may suggest that these lunar rhythms are due to effects other than from light, such as the moon’s maximal gravitational tug on the Earth at the new and full moons,” Dr. Casiraghi said.

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Leandro Casiraghi et al. 2021. Moonstruck sleep: Synchronization of human sleep with the moon cycle under field conditions. Science Advances 7 (5): eabe0465; doi: 10.1126/sciadv.abe0465