I have done several of these. When there is a full moon or you are above treeline it is no problem. Also a headlamp or two are a huge help. However, the fear and feeling of being alive and very aware are not to be missed. I just finished running through the Mines of Spain in the dark without a light. It is a well trod trail, but it is amazing how differently, how cautious, how sensitive we are when we are alone in the dark.