In modern times, much research has focused on how artificial light sources disrupt our sleep and health due to the unnatural effects of lighting after the sun goes down.
But how unnatural is night light really? After all, people are always exposed to variable light levels at night, due to reflections of sunlight from the waxing and waning moon – and this shifting radiation is stimulating us in ways we’re not fully aware of, new research suggests.
“Moonlight is so bright to the human eye that it is perfectly reasonable to imagine that this source of nocturnal light, in the absence of other light sources, would have played a role in modulating human nocturnal activity and sleep,” said one. team of researchers. , led by senior author and neurobiologist Horacio de la Iglesia of the University of Washington, explain in a new study.
But whether the lunar cycle can modulate human nocturnal activity and sleep remains a matter of debate.
To investigate the mystery, the researchers equipped more than 500 participants with wrist activity monitors to track their sleep patterns, and conducted the experiment in vastly different locations.
First, it involved 98 participants from the Toba-Qom people, an indigenous community living in Argentina’s Formosa province. Some of these participants in the rural experiment had no access to electricity, others had limited access in their homes, while a final contingent lived in an urban setting with full access to electricity.
In a separate experiment, the researchers tracked the sleep of 464 college students who lived in the Seattle area – a large, modernized city with all the electrified trappings of post-industrial society.
By monitoring the participants’ sleep activity throughout the lunar month cycle, the researchers found that the same kind of pattern could be seen in their sleep and waking no matter where the volunteers lived.
“We see a marked lunar modulation of sleep, with sleep decreasing and falling asleep later in the days leading up to a full moon,” says de la Iglesia.
“While the effect is stronger in communities without access to electricity, the effect is present in communities with electricity, including students at the University of Washington.”
While there was some discrepancy between the results, the data generally showed that sleep tends to start later and is generally shorter in the nights leading up to a full moon, when the crescent moonlight is brighter in the night. hours after sunset.
While the sample size studied here is not particularly large – and there is certainly more research that could be done here to expand these results – the same pattern was observed in two different populations living in different countries, and with such different levels of access to electricity between all the volunteers does tell us some important things, the team says.
“Together, these results strongly suggest that human sleep is synchronized with lunar phases, regardless of ethnic and sociocultural background, and the level of urbanization,” the researchers write in their paper.
As for the cause of these effects, the researchers argue that sustained nocturnal activity stimulated by moonlight could be an evolutionary adaptation carried over from the days of pre-industrial human societies – with the ability to stay up and do more under a beautiful full moon. take advantage of all kinds of traditional customs that are still enjoyed by those without electricity.
“At certain times of the month, the moon is an important source of light at night, and that would have been evident to our ancestors thousands of years ago,” said lead author and sleep biologist Leandro Casiraghi.
According to interviews with Toba / Qom individuals, moonlit nights are still known for high hunting and fishing activity, increased social events and increased sexual relations between men and women.
“Although the true adaptive value of human activity during moonlit nights has yet to be determined, our data appears to show that people – in different environments – are more active and sleep less when moonlight is available in the early hours of the night,” explain the researchers explain.
“This finding, in turn, suggests that the effect of electric light on modern humans may have seized an ancestral regulating role of moonlight on sleep.”
The findings are reported in Science Advances.