Nothing happens after the 18th hour, except that if you’re a normal human, you’re probably going to be going to sleep around then. (If you get up at, say, 7 am and get dressed right away, 18 hours later makes it about 1 am.) “18 hours” is about as much as they could get away with, marketing-wise, without implying that women don’t need sleep.
However, I would like to point out that I haven’t found a huge amount of difference, comfort-wise, between granny-style 18-hour ultra-supportive bras and my slinky cleavage-enhancing Victoria’s Secret bras, unless I’m working out. (Just for the record, I’m a 34D too…)