Sleep trackers monitoring nearly 100 people in both bustling metropolises and villages with little to no electricity revealed humans go to bed later and sleep less within a few days of a full moon.
(CN) — The moon has played an outsized role throughout human civilization. To the soothsayers of old, the moon was a sign of good fortune and bad. It promised hope and threatened despair. But more realistically, scientists now know it can affect how much sleep a person gets.
New research shows that oscillations in the lunar cycle may have a noticeable effect on a person’s sleep pattern. Even in brightly lit urban areas where the moon’s phase can only be detected by peering upwards, sleep patterns changed measurably in proximity to the full moon. Sometimes by up to 90 minutes.
Scientists from the University of Washington, the National University of Quilmes in Argentina and Yale University collaborated to study the phases of the moon and the effect it has on human sleep cycles, both in urban and rural areas. The team published their findings Wednesday in the journal Science Advances.
The researchers found that people go to bed later and wind up sleeping less overall in the days leading up to and following a full moon, when the sky is at its brightest around bedtime. These effects were less pronounced in bright urban environments where natural light is subordinate to the lightbulb, but the effect was still notable. They believe the universality in this pattern indicates humans’ natural circadian rhythms may have evolved in sync with the lunar cycle.
“We see a clear lunar modulation of sleep, with sleep decreasing and a later onset of sleep in the days preceding a full moon,” said Horacio de la Iglesia, a professor at the University of Washington and co-author of the study, in a statement. “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.”
The authors believe that in the days before electricity, people may have developed a propensity to sleep less under a full moon so they could get more done while light permitted. The waxing moon generally rises in the late afternoon to early evening as it becomes fuller, allowing work to continue later into the day. Particularly in hot regions, the ability to labor under a full moon is infinitely preferable to working under a scorching sun — so human sleep cycles may have evolved opportunistically.
“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 lead author Leandro Casiraghi, a postdoctoral researcher in UW’s biology department, in a statement.
To gauge the sleep cycles of individuals in both rural and urban settings, scientists fitted wrist-mounted sleep trackers on 98 people living in three Toba-Qom Indigenous communities in the Formosa province of Argentina. One community had no electricity at all, while a second community had only limited electricity, such as a single lightbulb at home. A third community located in an urban area had complete access to electricity.
Researchers gathered sleep data for 75% of each community’s residents over the course of one to two lunar cycles. A previous study by de la Iglesia’s team showed residents of the urban community routinely slept less and went to bed later than their rural counterparts, demonstrating the effect electricity already has on a person’s natural sleep cycle.
People in all three communities displayed the same changes in their sleep patterns across the 29.5-day lunar cycle. Bedtimes in each community were postponed by an average of 30 minutes, and residents slept an average of 46 and 58 minutes less within three-to-five days of a full moon.
Researchers compared the data gleaned from the Argentine residents with existing sleep data taken from 456 Seattle-area college students in a previous study and found the same pattern of oscillations exist even in a large, well-lit city.
“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 explained. “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.”