Bob Sutton recently assured the blogosphere that he is in good health. He had been receiving worried emails because he hadn't posted in almost a month. Here's his explanation:
The reason I have not been posting is that I have been hyper-focused on getting a complete draft of my next book done. For better or worse, when I get close to finishing something like that, I get so obsessed that I don't even realize how much I ignore other things.
I'm glad he qualified the strategy with "for better or worse" because there are real downsides to this approach to the final stages of a writing project.
Note that there is a difference between letting a manuscript occupy increasing amounts of your attention as you get closer to completing it and getting "so obsessed" that you begin to ignore other things (that you value) without realizing it. A writing plan can easily accommodate the former. The latter suggests the absence of a writing plan. Planning is the means by which you consciously (so that you do realize it) prioritize your activities. If you know you will think only about your book for a month, plan not to blog.
Likewise, in periods when you've got things to do that you can't ignore, don't plan to enter this "hyper-focus" mode. That means you should keep your deadlines (and ideally, this does not just mean your editor's deadline) away from periods in which you are doing a lot of teaching or fieldwork. In short, use planning to prioritize your attentions, don't focus on one thing to the detriment of others.
That said, writers need and deserve a bit of a magic circle around their work. Sometimes they need to allow themselves the luxury of dropping everything and immersing themselves in a particular text. They need to sit silently at dinner, brooding on their sentences, and take long walks to try to get things to fall into place. During this time, their friends, family and acquaintances might well ask them whether they are "feeling all right". In the olds days, this was called "melancholia" and was a perfectly accepted (if temporary) state of mind. Maybe "hyper-focus" is a fitting magic word. Invoking it secures a bit of space, a freedom from interruption, and a certain amount of license for anti-social behavior (like not blogging, or, I guess, updating your Facebook profile). It even rhymes with hocus pocus.
Just don't, if you'll pardon it, believe your own hype. Don't begin to romanticize your obsessive process or your disregard for other things. Like all magic, it's a trick. There is a more natural, more efficient, and less dramatic process that could explain how you completed your manuscript too.