I always carry my sleep sheet. I don't need it that often but I'm always glad I have it. If you don't have one, you can save money by making one -- just take an old and sew it up on three sides. It's bigger that way too.
I think almost all hostels provide sheets now, don't they? A few times I've had to pay extra for one, but they always have them. I think it's kind of gross to carry around the same smelly sheet!
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