File this one under "things no one tells you about" when you become a new mom: sleep regressions. Just when you thought you'd gotten your baby to sleep through the night, he rocks your world with persistent, unavoidable night wakings. Sigh . . . It's not a fun time for anyone, but it's reality, and we're here to tell you that (a) it's normal and (b) this, too, shall pass. Here's what to expect and how we've learned to cope.