nycti-, nyct-, nycto-, nyc- +
(Greek: night; a relationship to darkness, dark)
The tendency of certain plant organs, as the leaflets of clover, to assume special “sleeping” positions at night.
2. An abnormal preference for darkness or night.
Nyx lived in Tartarus, from which she went forth each day just as Hemera was returning. Her only other recorded act was to save her son Hypnos when Zeus was about to throw him out of the realm of the gods.
Tartarus, by the way, was a dark region beneath the earth, and the personification of that region. Tartarus was said to be as far beneath Hades, or beneath the surface of the earth, as heaven was above it. Supposedly, an anvil would fall for nine days to reach it from earth’s surface.