Whoa. Is this the law, or recommendation? More than half of my flights have been AFTER dark. Orientation is easier in the dark with green lights in back and the red arms lit up in front. Wind is calmer. Also, reception is better after dark. The ionsphere does not interfere with the RC/Video transmission in the evening. Radio stations typically lower their output at night because there is no solar UV radiation, and reach the same listneners, usually more.
I'd like to know because I enjoy flying at night.
SD