Under no circumstances should you drive Saddle Road at night.

If you have the time, I'd consider taking the southern route outbound with maybe a stop at the Punalu`u Black Sand Beach to see the turtles (and the Ka`u Desert is awesome in its own way), and the northern route for your return. If you've driven on the Big Island before you know a couple of things: a) at night it is DARK. If there is any decent rain happening -- a good chance on some portion of the trip between Honoka`a and Hilo -- slow down and be alert in the "cuts" to any debris on the road; and b) the mainland "conservative" rule-of-thumb of 50 miles every hour readily goes out the window on any drive on the Belt Highway; sometimes 30 miles in an hour if in the rain and/or behind slow trucks, etc., is the best one will do.