Humans are diurnal – we are active during the day and sleep at night. Our circadian rhythms are in sync with the sun, the strongest source of light on the planet. However, recent evidence suggests that moonlight may also influence our sleeping patterns, and this effect changes with the lunar cycle.

"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," said Leandro Casiraghi, lead author of a new study, published in Science Advances.

The research, a collaboration between the University of Washington, Yale University, and the National University of Quilmes in Argentina, assessed the activity patterns of 562 participants across an urban-rural gradient. Participants included members of three Indigenous Toba/Qom communities in Argentina and undergraduate students from the University of Washington. Each participant slept with a wristwatch that tracked their movement and sleep. This data was combined with NASA's sun and moon data for each location.

The researchers found that the time the participants went to sleep and how long they slept oscillated during the course of the lunar cycle. On nights leading up to a full moon, when the moon was brighter, people went to sleep later and slept for a shorter time. 

The researchers concluded that moonlight stimulates nocturnal activity, especially for those in rural communities where light pollution is sparse. People in cities, with greater light pollution, went to sleep later and slept for less time in general, but still followed similar patterns to people living with little or no access to artificial light. The researchers state that the artificial light present in cities mimics the effect that moonlight has on sleep for rural communities. 

So next time you are having trouble falling asleep, you may want to look to the moon for an explanation.