For centuries, humans have blamed the moon for our moods, accidents and even natural disasters.
But new research indicates that our planet’s celestial companion impacts something else entirely — our sleep.
In a new study, researchers found that sleep cycles in people oscillate during the 29.5-day lunar cycle: In the days leading up to a full moon, people go to sleep later in the evening and sleep for shorter periods of time.
They found these variations in both the time of sleep onset and the duration of sleep in urban and rural settings — from Indigenous communities in northern Argentina to college students in Seattle, a city of more than 750,000.
They saw the oscillations regardless of an individual’s access to electricity, though the variations are less pronounced in individuals living in urban environments.
The pattern’s ubiquity may indicate that our natural circadian rhythms are somehow synchronized with — or entrained to — the phases of the lunar cycle.
The research was conducted by scientists at the University of Washington and elsewhere.
In the study, the team used wrist monitors to track sleep patterns among 98 individuals living in three Toba-Qom Indigenous communities in the Argentine province of Formosa.
One rural community had no electricity access, a second rural community had only limited access to electricity — such as a single source of artificial light in dwellings — 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.
The team found the participants in all three communities 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 team analyzed sleep-monitor data from 464 Seattle-area college students that had been collected for a separate study. They found the same oscillations.
The team confirmed that the evenings leading up to the full moon — when participants slept the least and went to bed the latest — have more natural light available after dusk:
The waxing moon is increasingly brighter as it progresses toward a full moon, and generally rises in the late afternoon or early evening, placing it high in the sky during the evening after sunset.
The latter half of the full moon phase and waning moons also give off significant light, but in the middle of the night, since the moon rises so late in the evening at those points in the lunar cycle.
The researchers hypothesize that the patterns 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.
One author of the study is UW professor of biology Horacio de la Iglesia.
The study is published in Science Advances.
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