There’s something special about moonlight. We often think of it as soothing, mysterious and even romantic. But could it play a role in sleep quality?

Along with the phases of the moon, humans’ sleep waxes and wanes. Researchers have determined that people stay up later when moonlight is strongest. “Sleep starts later and lasts less on the three or five nights that are before the full moon. On those nights, sleep tends to be shorter,” said senior researcher Horacio de la Iglesia, PhD,a professor of biology at the University of Washington in Seattle. His research has revealed that the best sleep appears to occur during the few nights immediately preceding the new moon when there is the least amount of moonlight.

Researchers tracked sleep pattern data from more than 450 students at the University of Washington. Their analysis indicated that everyone’s sleep cycles fluctuated with the phases of the moon. The total amount of sleep varied based on association with the lunar cycle, with average fluctuations of between 46 and 58 minutes. Bedtimes varied by about 30 minutes.

Researchers found the results of their analysis surprising, de la Iglesia said, since there was little reason to believe individuals in urban environments would be aware of the moon phases, whether full, new, half or quarter. Study results were published in the journal Science Advances.

For quite a while it’s been recognized that light plays a critical role in human circadian rhythms, according to Steven Feinsilver, MD, the director of the Center for Sleep Medicine at Lenox Hill Hospital in New York City. Although he was not involved in the study, he explained, “There are specific cells in the eye that sense blue light, which were not known of until about 20 years ago.” Referring to the light emanating from televisions, computer screens and smartphones, he said of these blue-light-sensing cells in the eye, “They’re not very involved in vision, they don’t do much for vision, but they send signals to the brain saying ‘lights are on, time to be awake,’ ‘lights off, time to be asleep.’”

Progressing toward a full moon, the waxing moon grows increasingly brighter, typically rising in the late afternoon or early evening and appearing high in the sky during the evening after sunset, according to researchers. In order to work or socialize later into the night, de la Iglesia suggested that early human sleep cycles might have adapted to take advantage of the illumination provided by moonlight. And it now appears that it has developed into an inherent trait in humans. “That’s exactly what we do with artificial light,” he said. “We extend the end of the day. And that’s exactly what we saw people do with moonlight.”

These new findings could have important practical implications for individuals experiencing sleep problems, according to de la Iglesia. “If your doctor gives you a list of practices you should follow to improve your sleep starting—like don’t get exposed to bright light, avoid screens, avoid highly stimulating media—maybe on the nights that precede the full moon you could be a little more proactive with those measures because you know ahead of time that those nights are going to be particularly harder,” he said.

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