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  • A new study finds we sleep less on the nights leading up to a full moon, but researchers don’t understand why this occurs.
  • Researchers looked at people who live with no, limited, and full access to artificial lighting to find the same changes in sleep as the lunar cycle progresses.
  • Experts are still trying to understand why these sleep changes may happen.

We’ve all heard stories of how things can get a little strange during a full moon, from increased rates of psychiatric hospitalization to a higher risk of being bitten by an animal.

However, findings from a study published Wednesday, Jan. 27, suggest that the lunar cycle does affect how you sleep.

Using wrist monitors, researchers tracked sleep patterns among 98 people living in three Toba-Qom indigenous communities in Argentina.

“For the last few years we have been studying Toba-Qom that live in communities with different levels of access to electricity,” lead researcher and University of Washington professor of biology, Horacio de la Iglesia, PhD, told Healthline.

These communities differed in access to electricity during the study period:

  • One community had no electricity.
  • Another had limited access to electricity, such as a single source of artificial light.
  • A third community lived in an urban setting with full access to electricity.

Researchers collected sleep data for one to two lunar cycles for about 75 percent of the Toba-Qom participants.

Researchers found participants in all three communities showed the same sleep pattern changes as the moon progressed through its 29.5-day cycle.

On average, people went to bed latest and slept the least 3 to 5 days before a full moon.

Then they analyzed sleep-monitor data from 464 Seattle-area college students collected for a separate study to find the same pattern of sleep changes.

“Although we had hypothesized that sleep would be inhibited during moonlit nights, we were particularly surprised by two findings,” de la Iglesia said.

“First, we did not see a maximal inhibition of sleep exactly during the full moon nights, as we had predicted; instead, nocturnal activity increased and sleep was shortest starting a few nights before the night of full moon,” he said.

De la Iglesia said he initially believed this was because more moonlight is available during the first half of the night, but not necessarily the nights that follow the full moon (because the moon rises later every night).

“Second, we were extremely surprised to find that the effect, although smaller, was present regardless of the access to electricity, and in fact, even in university students living in Seattle!” he said.

Dr. Alex Dimitriu, who is double board-certified in psychiatry and sleep medicine, said the moon most likely exerts its effect by an increase in evening or nighttime light. This might suppress melatonin (a sleep hormone), which affects the onset and duration of sleep.

“According to this study, it does seem that there is a significant delay and decrease in total sleep time on nights leading up to a full moon,” said Dimitriu, founder of Menlo Park Psychiatry & Sleep Medicine.

“We know that light, whether artificial or natural — from the moon, or the setting sun — can have a suppressant effect on melatonin,”he said. “So it is plausible that moonlight could have a natural wake promoting effect.”

De la Iglesia said the main limitation of the study is that they can’t establish a causal link between moon phase and the changes in sleep.

“Obviously, sleep timing is synchronized with the moon phases, but we still do not know how this happens,” he said.

But he thinks the moon’s gravity might explain it.

“We believe that the gravitational pull cycles associated with the lunar month may predispose humans to be particularly sensitive to the effects of light, moonlight, or artificial, on the nights close to the full moon,” de la Iglesia said.

“The point of all this is humans are really light sensitive,” said Dr. Steven H Feinsilver, director of the Center for Sleep Medicine at Lenox Hill Hospital in New York.

“We all have a circadian rhythm, a built-in body clock, it doesn’t necessarily run a 24-hour cycle, and probably runs slower in most people — a 25-hour cycle,” he said.

It’s exposure to light that trains us into a normal 24-hour cycle, he said, and that “light is the thing that really turns your brain on.”

Feinsilver was unsure about lunar gravity having any appreciable effect on sleep.

“We can talk about that it’s possible it’s not light,” he said, “but the gravitational pull of the moon, and that’s a lot harder to understand, there’s not even a biological known basis for that.”

According to Dimitriu, a sleep loss of 20 to 30 minutes is generally well tolerated in someone who ordinarily gets about 7 to 8 hours of sleep per night.

However, he cautioned that this can become problematic for people who sleep fewer than 7 hours on average or who don’t usually sleep well.

“Healthy sleepers will most likely do just fine with a minor decrease in their total sleep time. For people with insomnia, thin, or unrefreshing sleep, a loss of 20 minutes can be adding insult to injury,” he said.

Dimitriu said that modern life, with its artificial light sources, and forms of entertainment like smartphones and TVs, “likely exerts a far greater effect on our sleep than moon phases.”

This puts the emphasis on maintaining “healthy sleep behaviors and bedtime habits,” he said.

A new study finds we sleep less on the nights leading up to a full moon. However, researchers don’t understand why this occurs.

Researchers looked at people who live with no, limited, and full access to artificial lighting to find the same changes in sleep as the lunar cycle progresses. They think the moon’s gravity might have something to do with it.

Experts say there’s still no evidence that lunar gravity can affect sleep, and that light in some way is likely causing this effect.