Phases of the moon could be playing with your bedtime without you knowing it, study suggests

New science hints that Earth's nearest neighbour may be influencing human behaviour in a small, but significant way, by affecting our sleep cycles.

Sleep researcher Leandro Casiraghi's findings suggest sleep cycles are synchronized with the moon's phases whether we're aware of the moon's phase, or even see it. Exactly why this is happening is unknown.

"We were kind of shocked," said Casiraghi, a postdoctoral researcher in biology at the University of Washington. 

He is the lead author of a study published this week in the journal Science Advances, which tracked three Indigenous communities in Argentina, and compared them to students at the University of Washington. The work was led by University of Washington biology professor Horacio de la Iglesia and involved a team of researchers from Yale University and National University of Quilmes in Argentina.

The team fitted participants with wrist monitors akin to sophisticated smart watches, which collected biological sleep data for between one and two lunar cycles (one lunar cycle lasts about 29.5 days.)

A majority of the participants in the study were found to go to bed later and sleep less in the week ahead of a full moon.

"It might be that the biological clock is being delayed in a way that now it's telling you to go to sleep at a later time, but it might also be that for some reason this accumulation of waking time is not as important during certain days of the moon cycles," said Casiraghi.

Sleep in the real world

Casiraghi's research supervisor de la Iglesia had previously studied how lunar phases are correlated with increased activity in animals, such as crabs and monkeys in Argentina.

That was a springboard to asking: What about humans?

While previous sleep clinic research had investigated connections between lunar cycles and sleep, Casiraghi said those studies were limited by the fact that participants were in an unrealistic, lab-like setting, and results couldn't be easily replicated.

Casiraghi noted past research has established that with greater access to electric light, people tend to go to sleep later into the evening and have shorter nights than those with limited access.

He set out to study this in the real world, tracking the sleep patterns of about 100 participants spread across three Indigenous Toba/Qom communities in Formosa, Argentina.

One community was urban and had ready access to artificial light; another was rural but had limited access to artificial light; and the third, also rural, had no access to artificial light.

Casiraghi said going into the study, the team assumed if they found any sleep effect linked to lunar phases, the signal would be strongest on the night of a full or new moon, when the moon glow was at its brightest in the middle of the night. The argument would be that the light helped our ancestors see more clearly and stay active later.

Assumptions eclipsed

That's also why the team assumed they might find an effect in the groups with no or little access to electric light, because those groups might stand to benefit more from moonlit nights than those with ample access to light in the home.

But two unexpected patterns emerged.

The team found most people across the three groups went to bed later and slept less than usual — not on a full or new moon — but in the three to five days before. The amount of sleep on average varied between 46 and 58 minutes over the course of the lunar cycle. Bedtimes varied by around half an hour.

And sleep cycles of the urban and two rural communities displayed similar rhythm changes in the few days before the full moon, regardless of access to light.

The urbanites with access to light around the clock went to bed later and slept less on average than either rural community during those days. The rural community with some access to light went to sleep earlier than the urbanites, but later than the light-free rural community, which also slept more than all three. 

But the main effect was the same. Access to electric light didn't shield groups from the stimulating effects of the moon.

"We went through the data like 10 times to convince us what we were seeing was actually true," said Casiraghi.

The team compared their findings to sleep data gleaned from nearly 500 University of Washington undergraduates in past studies. 

The same "striking" correlation between moon phases and sleep patterns was apparent in the undergrads living in post-industrial Seattle as in the Toba/Qom.

Light not the only driver

Casiraghi has a theory as to why it would've benefited early humans to stay up later during this time.

In the days before a full moon, the sun sets and a luminous moon takes its place in the sky earlier in the evening than usual. 

Casiraghi suggested it could've been adaptive for hunter-gatherer groups to take advantage of the fact that the moonlight "stretched the day," allowing longer periods of activity. Full moons in the middle of the night provided less useful benefit. 

This squares with the cultural stories of the Toba/Qom. They said brighter nights around the full moon were traditionally used to safely hunt and fish later into the evening. It was also a time for greater social activity, they say — including romantic trysts between young people.

But Casiraghi admitted this doesn't fully explain what's going on.

If people living in a big city setting surrounded by television screens, household light sources and light pollution are staying up later ahead of a full moon, the light itself can't be the only driver.

The answer may be hiding in our biology. 

On any given night, we tuck in for two reasons: our biological clock tells us to, and we've been awake so long we've reached an energetic limit for the day.

Which of those two is being influenced by the moon — and how the moon is exerting that influence — remains a mystery, said Casiraghi.

"These are the questions we're trying to engage with in the near future." 

Written and produced by Bryce Hoye.