Moon and Sun aligned. The lit face turned away from Earth. With no visible glow at night, the sky stays darker for two or three nights in a row. That changes the amount of light reaching your eyes before sleep, and in people who are more sensitive, it shifts melatonin.
The pattern
Less drive to show up. Less social energy. More need for quiet. This account shows up across very different cultures, very different centuries, very different words. Enough that New Moon rituals exist in civilizations that never had contact with each other.
Nothing mystical about it. Less ambient night light, different sleep, different behavior.