I came across a post by Paul Graham a month or two ago that really made me think and I meant to write something about it but never got round to it. (Read it here).
The basic premise is that whatever is the main focus at the moment will take up all your spare thought processing time, potentially at the detriment of other things. His example is for a start-up that has grown to a certain size and is looking for funding and this becomes the focus to the detriment of all the other activities they had been doing to that point. This is not a concious choice but the thought of fund raising takes all the spare thought time that previously would have been for other things.
I have encountered this so many times in both a positive and a negative way. The positive way has happened when I am completely consumed by a problem, typically at work, and eventually a solution will arrive or it will lead to something else.
On the negative side it can certainly be a massive drain on brain power to be thinking about less productive and potentially less important things.
We moved house recently and this has definitely been the "top of the mind" idea. Every spare moment was filled with either the logistics of moving or locating a new house or finances etc. I did notice that it impacted other areas especially the creative side of thinking about issues and also relaxing.
Now I try to catch myself when the focus gets too much on one thing and deliberately spend some time focussing on other things - sometimes it works, sometimes the old idea just creeps back in - but I am certainly more conscious of what is happening.