When my son was a little over a year and a half old, he started to notice day/night cycles explicitly but he recognized them as "light and dark". One night, at dusk, I saw him standing by the window, pressing his hands against the glass, looking out into the night. It was a gloomy night with very dark purple skies and silhouetted black branches waving in a heavy wind.
I asked him what he was looking at. He turned to me and smiled as he said, "Dark is here."
The shudder that ran through me still make me look over my shoulder whenever I remember this as night falls and I find myself alone in a room.