"There are two types of schedule," he writes at paulgraham.com, "which I'll call the manager's schedule and the maker's schedule. The manager's schedule is for bosses. It's embodied in the traditional appointment book, with each day cut into one-hour intervals. You can block off several hours for a single task if you need to, but by default you change what you're doing every hour.
"When you use time that way, it's merely a practical problem to meet with someone. Find an open slot in your schedule, book them, and you're done . . . But there's another way of using time that's common among people who make things, like programmers and writers. They generally prefer to use time in units of half a day at least. You can't write or program well in units of an hour. That's barely enough time to get started. When you're operating on the maker's schedule, meetings are a disaster. A single meeting can blow a whole afternoon, by breaking it into two pieces each too small to do anything hard in."
Each type of schedule works well enough on its own, Graham says. "Problems arise when they meet. Since most powerful people operate on the manager's schedule, they're in a position to make everyone resonate at their frequency if they want to."
The rest of the article is also worth reading, but these 3 paragraphs are exactly what I've seen myself struggling with recently, especially when I've been attempting to do both a management (day to day line management and team management) and a making role (active development on my current project) at the same time. I've taken to blocking out entire afternoons to enable me to get on with "making". This article has given me the justification to continue doing that and I might just try and block book all afternoons to see what I can achieve without meetings cropping up here and there.