Obviously it wasn't individually coiled springs topped with 6 inches of memory foam.
EDIT - Sorry, should clarify, I asked about those in this century with the sacking and fall of Rome in mind. So basically, what would a typical Roman have slept on. And if its known, how would that have differed from how the invading tribes would have slept, if at all.
Can you be more specific about place? An average person living in Teotihuacan is different than one living in Athens or Jiankang. Remember, saying around the world breaks our rules.