In the nights leading up to a full moon, people tend to do this

Just a few days ago 2021 saw its first full moon. On a related note, if you noticed that you hadn’t been sleeping very well last week a new study just released by the University of Washington finds your disrupted sleep patterns may have something to do with the lunar phases of the moon.

Researchers have collected the most compelling evidence to date indicating that during the nights just before a full moon people tend to go to bed later and wake earlier. This lunar effect appears to be stronger among those living in rural areas with less access to electricity, but even city dwellers with 24/7 internet access also saw their sleep habits suffer just before a full moon.

“We see a clear lunar modulation of sleep, with sleep decreasing and a later onset of sleep in the days preceding a full moon,” says study leader Horacio de la Iglesia, a professor of biology at UW, in a release. “And although the effect is more robust in communities without access to electricity, the effect is present in communities with electricity, including undergraduates at the University of Washington.”

Why does the buildup toward a full moon impact our sleep to such a degree? Well, the waxing moon slowly grows brighter and brighter with each passing night as it slowly cycles toward the full moon phase and also tends to rise higher in the sky earlier in the evening. All of this results in a whole lot more natural light at night. So, researchers theorize this tendency for humans to stay up late and sleep fewer hours on bright nights is an evolutionary holdover from long ago. 

Thousands of years ago, the nights before a full moon would have been an advantageous time for early humans to catch up on whatever needed doing.

“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,” explains lead study author Leandro Casiraghi, a UW postdoctoral researcher in the Department of Biology. “At certain times of the month, the moon is a significant source of light in the evenings, and that would have been clearly evident to our ancestors thousands of years ago.”

Researchers from the National University of Quilmes in Argentina and Yale University also collaborated on this project.

The moon goes through just over 12 lunar cycles per year, completing each cycle roughly every 29.5 days. To track the impact of lunar cycles on sleep patterns, researchers analyzed sleep onset and duration fluctuations among a varied population sample. First, 98 individuals living in the Toba-Qom Indigenous communities of the Argentine province of Formosa were fitted with sleep tracking wrist monitors for one to two lunar cycles. Importantly, those Argentinian study subjects all hailed from one of three different communities. One group had no access to electricity at all, another had limited access to light/electricity, and the third lived in a more urban region and had full access to electricity.

While the Argentines living in more urban areas, or homes with better access to electricity, went to sleep later and woke earlier pretty much year-round, that’s a documented side effect of modern living. Our ancestors didn’t have the luxury of lamps, laptops, and TVs. When the sun went down there wasn’t all that much to do, so humans generally slept more. 

But, beyond these expected sleep differences between urban and rural populations, all studied subjects showed noticeable changes in their sleep patterns in the three to five nights preceding a full moon. Across all three studied communities, participants’ sleep duration fluctuated by an average of 46 to 58 minutes, and usual bedtimes changed by around 30 minutes.

To corroborate these findings with a completely unrelated dataset, the study authors then decided to analyze sleep-monitor data collected among 464 Seattle-area college students. Once again, these students showed later bedtimes and less sleep duration just before a full moon.

These findings are quite fascinating in several ways, but on an even grander scale, the research team wonders if they’ve inadvertently explained why sleep is so hard to come by in modern times – regardless of the shape of the moon. If extra moonlight at night tells our brains and bodies to stay awake, just imagine what effect artificial light from technological devices is having on circadian rhythms. 

“Artificial light disrupts our innate circadian clocks in specific ways: It makes us go to sleep later in the evening; it makes us sleep less,” notes professor de la Iglesia.

At the very least, this study makes for a great excuse the next time you end up staying up too late the night before a big day. You’re not to blame; it was the full moon’s fault!

“In general, there has been a lot of suspicion on the idea that the phases of the moon could affect a behavior such as sleep — even though in urban settings with high amounts of light pollution, you may not know what the moon phase is unless you go outside or look out the window,” Casiraghi concludes. “Future research should focus on how: Is it acting through our innate circadian clock? Or other signals that affect the timing of sleep? There is a lot to understand about this effect.”

The full study can be found here, published in Science Advances.