Crepuscular

crepuscular \kri-ˈpəs-kyə-lər\ adjective: 1. of, relating to, or resembling twilight; dim {crepuscular light}, 2. occurring or active during twilight {crepuscular insects} {crepuscular activity}

Etymology: Latin crepusculum, from creper dusky

The noun form is crepuscule. According to Merriam-Webster it’s a straight synonym for twilight, but to my ears crepuscule sounds like a portmanteau of crêpe and corpuscle. That would make it a small, thin pancake of cellular bodies, perhaps blood cells.