Researchers studied people from Indigenous communities in northern Argentina to college students in Seattle. They saw the oscillations regardless of an individual's access to electricity.
The pattern may indicate that our natural circadian rhythms are somehow synchronized with -- or entrained to -- the phases of the lunar cycle.
In the evenings leading up to the full moon (when participants slept the least and went to bed the latest), there is more natural light available after dusk. The moon is increasingly brighter as it progresses toward a full moon, and generally rises in the late afternoon or early evening, placing it high in the sky during the evening after sunset.