To begin, I point you to 210.11(C)(3), which says: "...at least one 20-ampere branch circuit shall be provided to supply the bathroom receptical outlet(s). Such circuits shall have no other outlets."
So it would seem that I can't supply my lights from the same circuit. But wait! There's the following exception in the next paragraph: "Exception: Where the 20-ampere circuit supplies a single bathroom, outlets for other equipment shall be permitted to be supplied in accordance with 210.23(A)."
In 210.23(A) I find: "A 15- or 20-ampere branch circuit shall be permitted to supply lighting units or other utilization equipment, or a combination of both..."
The exception makes it appear that, because I'm supplying a single bathroom, I indeed can supply both the lights and the outlet. However, there's an exception to the exception! Under 210.23(A), I also find: "Exception: The small appliance branch circuits, laundry branch circuits, and bathroom branch circuits required in a dwelling unit(s) by 210.11(C)(1),(2), and (3) shall supply only the recepticle outlets specified in that section."
I'm now in an circular loop. The exception in 210.11(C)(3) points me to 210.23(A), and then the exception in 210.23(A) points me right back to 210.11(C)(3).
Do I not follow the second exception? Does code allow me to service all of a single bathroom with a single circuit? It should we be convenient. One room/one circuit makes it much easier to label the breakers.