what I do with the 5/6am feed (I bf) is offer a smaller one & then feed upon waking & then adjust the day tiem feeds by small amounts.
If you ff, maybe try offering less than normal at the earlier feed?
Our first feed is actually about 8am & so for me it is the 6/7am one that is the problem, so I would say feed only one side at 6, then feed at about 8.30, then 11.45 (rather than normal 11.30) then I'm back on track.
That all said, I have a 2yo so often our feeds can be up to 1/2hour out of sync... we just roll with it & work towards the late afternoon feeds being at the time we want them.
i don't df either, as I'd rather go to bed early.