One phenomenon I have seen with otherwise productive writers is when they get their rhythms interrupted, and they fall rapidly farther behind. It's worth noting that it is possible to lower one's goals to suit the circumstances.
Charlaine Harris took off five years after she had kids, and she's doing okay now (I recommend her Sookie Stackhouse vampire mystery series highly). Other people I know took off a summer, which also captured a natural rhythm of "going back to school," in one case because she was going through a divorce and a motive, and it was just too much.
A couple of things you should do, however, to make it work better. One is to set a formal deadline, one that is meaningful to you--which is why the "going back to school" thing was such a good idea. It needs to matter if you miss your goal.
I've seen some people say it is too bad that NaNoWrMo is only once a year. On the other hand, it prevents people from saying "well, I'll just do the next one." Wait a whole year? But that's too long!
People used to watch certain television shows at certain times, because prior to videotapes, TiVo, cable, satellite and DVDs, all you had to watch was what the networks chose to show you. Watching The Wizard of Oz on Thanksgiving (American reference, I know. Any local equivalents?) was an event, one you looked forward to. I used to watch How the Grinch Stole Christmas every year (yes, even before I had kids), but now I can watch it anytime--and sometimes I don't! So we have our Christmas tapes and DVDs in the attic, and bringing them down is an event. For less than a month we have our Christmas tapes and whatnot, and so we only watch the brilliance of the Grinch, The Year without A Santa Claus (Snow Miser, yeah!), and A Pinky and the Brain Christmas for a limited time.
"For a limited time" -- doesn't that arouse some feeling in you? It should--it's that darn Yerkes-Dodson Law thang creeping up. Setting constraints raises the energy around it--you have a limited window. Similarly, NaNoWrMo pumps people up, while not setting overly high expectations of the outcome. One writer I know sets a "zeroth draft" in which he expects to be bad. He isn't necessarily, but he wants to keep his internal editor at bay. No one expects a good novel to come all at once in a mad rush, Dr. Jeckyll and Mr. Hyde notwithstanding.
So if you do take a break, make sure you draw sharp lines around it, which you yourself are unwilling to cross. You may find yourself excited to restart--which is exactly what you want!