Even in December I was in and out of my sleeping bag all night. Got hot, got cold, got hot, got cold. I cant remember any single night where I was rugged up in my sleeping bag all night. Most Albergues had some sort of heating, but it tended to go off after midnight. Even so, I had no real issues, but was glad to have sleeping bag if I needed it. You can sleep in it, or on it, depending on the Albergues and the heating situation, and how close your bunk is to the heating. I do know of one pilgrim who sent his sleeping bag home to the Netherlands at about Pamplona, and he regretted it on quite a few nights after that as we headed into December. He was relying on the Albergues having blankets and that didn't always pan out to keep him warm enough.