I'm debating whether I should skip the sleeping bag and only bring travel sheets. I have a small sleeping bag that only weighs 500g but it doesn't give much more warmth than a blanket.
This sounds like my sleeping bag. I would bring it if I were you. The less you have to rely on supplied blankets and pillows the better. If you're really cold, well than most albergues have blankets (not always for all beds) that you can put over top of your sleeping bag. You don't need a pillow - roll up your clothes in your fleecy or jacket instead, or use a pareo or sarong to enclose your clothes into a pillow.
I haven't so far, but based on others' postings, I think I'll bring one next time. There were a few times it was pretty chilly and I slept in my sleeping bag in all my clothes. A liner would have helped, and the weight is negligible.
I didn't stay in the albergues and all the small pensions used gave sheet and pillow case and some sort of cover but I used my sleeping bag a lot for extra warmth. Some places gave towels as well, but not all.
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