As I’m reading Shape Up, this has perfectly captured my perspective of interruptions:

When people ask for “just a few hours” or “just one day,” don’t be fooled. Momentum and progress are second-order things, like growth or acceleration. You can’t describe them with one point. You need an uninterrupted curve of points. When you pull someone away for one day to fix a bug or help a different team, you don’t just lose a day. You lose the momentum they built up and the time it will take to gain it back. Losing the wrong hour can kill a day. Losing a day can kill a week.

It is the perspective of which I’d like to view interruptions. The cost of a single interruption is multiplied by the number of time units you have been focused in. When working for 6 hours continuously, the interruption would destroy everything you have mentally built for 6 hours. You need to rebuild (accelerate from zero) to get back to the state of deep focus you were in.

Acceleration and momentum is the physical metaphor Ryan has used. I usually think of deep working as riding a bicycle, you start from zero, then start accelerating till you reach the maximum speed. If someone down the road stopped you (aka: interrupted you) for a minute, you need to settle down and then start accelerating back from zero. Multiple interruptions during the day = you lost the entire day.