There are a few theories as to why a full moon might have a negative impact on sleep. One popular one is that it's to do with the increased amount of moonlight (that was the conclusion of the large-scale 2021 study mentioned above). Moonlight is sunlight bounced off the moon's surface and reflected back to Earth – albeit much more weakly. Because we know that our sleep-wake cycles are stimulated by rising and falling hormone levels in response to light, it makes sense that this could cause disrupted sleep.