Un-interrupted time is important. But I think you're getting at something else I've noticed, that I have to already know what to do in the un-interrupted time.

Un-interrupted time is important. But I think you're getting at something else I've noticed, that I have to already know what to do in the un-interrupted time. Just having a chunk of time is not enough for high productivity. Already-knowing-what-to-do can come from moments of insight in the shower or walking the dog, or can come from state dumps -- written to-do notes -- made after I've loaded sufficient state on an earlier day. Sometimes both knowing-what-to-do and the time to do it can occur in the same session. This tends to be smaller stuff. And sometimes, after taking that shower the next day, it turns out to be the wrong stuff.