The 18 Hour Bra and other mysteries

When I was little I used to always see commercials for "Playtex: The 18 Hr Bra" and wonder what that meant. Do other bras last less long? Do they fall apart under the sheer weight of your breasts after too long?

Why not 24 whole hours? Well, when I started wearing a training bra I realized that you don't have to sleep in the bra. So that made a little more sense. But am I supposed to only sleep braless for six hours?

I thought that at some point I'd simply become a grown up and that this would finally make sense to me. Like I'd be sipping coffee in a cafe in Europe, because that's what adult women do, and all of a sudden *DING*, "Ohhhh. 18 HOURS." and then I'd smile a smug little smile and switch my reversible skirt to the sparkly side and go out to dinner with my boyfriend.