This is normal as the wall is cold and there is moisture in the inside air. One thing you can try is put a foam pool noodle from the dollar store between the mattress and the wall to allow some circulation. You should also check under the mattress as the moisture from your body will get in the mattress and then condense when it hits the cold surface. To correct the under mattress issue, add a piece of 4X8' foam siding insulation ( with aluminized facing) under the mattress.
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