Searching thru the net the answer is a cloudy answer.....The federal government doesn't have laws about hours between shifts. However, state rules and industry rules do vary. The stuff I saw mentions that some industries: hospitality, tourism, retail, agriculture don't have any specific laws. These jobs are highly driven by customer demand so you can possibly work two shifts in a row that are less than 8 hours apart.
There are some industries highly regulated on how many hours a person can work.....the airline industry and trucking companies are just a couple of examples.
So I guess that there really isn't any clear cut laws about time between shifts...and even if there were the retail industry doesn't seem to have to follow the same rules.