I'm lucky, I've never had it. What I've observed is that good days or bad days are just perception. When I've looked back on what I've written, my style, cadence, and quality, tends to remain the same. So I'd recommend less thinking and just grinding through. The great thing with a novel (versus a screenplay) is that you can toss out 50 pages and MAKE IT BETTER. So even if what you write is meandering, or in the end, not needed, who cares?