What is the primary difference between the current sleep practices in the United States and historic sleep practices?
I assume that you are asking about sleep practices with regard to babies. If so, the primary difference between current and historic sleep practices is that people now almost always have their babies sleep away from the mother. Historically, infants would more generally sleep with their mother, but we have moved away from that, largely because of fear of smothering children by rolling over on them.
If you are talking about sleep more generally, the primary difference between sleep today and sleep in history is that we stay up later than we used to. This is particularly true of what was done in the (relatively recent) times before everyone had electric lights. People now stay up much longer than we "naturally" would if we did not have electric lights and things like TVs to keep us up late.