[zipdog]

Yesterday a friend explained the physiological reasons behind 25 minute breaks: basically the muscles (including eye muscles) go into a different state of relaxed-tension and its generally not good for them to be in that state while the body is sitting-looking.

There's also the issue of the effort required to concentrate on a task, which gives diminishing returns as you extend it beyond ~25 minutes without break.

However a mental flow state (what you seem to describe) requires little to no concentration effort and so can yield great returns over the time spent. The tradeoff is that the body is in a less-than-desirable muscular situation for a prolonged time - but I'm guessing the damage there is not so significant if 8 hour flow sessions are not a daily habit.

tl;dr: the 25 minute break thing is probably a good general guide, but shouldn't stop you pulling an 8 hour session if you are absorbed and productive