To make the sponge: In a large bowl, whisk together the flour, bran, water, and yeast. Cover the bowl with plastic wrap and let sit at room temperature for 30 minutes. Refrigerate overnight. Bring the sponge to room temperature for at least 1 hour before mixing. Alternatively, after mixing the sponge, leave it at room temperature for at least 2 hours or as long as 6 hours before proceeding to the next step.