Cloning is not exactly 'natural'...
I clone under 24 hours of light with fine results.
Personally, I think that 24 hours is far safer because it is important to keep the temperature and %RH steady under the prop dome, and this is a serious pain in the ass to tweak if you are actually running dark time. You DON'T want dewfall on your clones. It's a good way to encourage mold.