Something like the alarms that wake you up at the right point in your sleep cycle.
It would probably need to have a cut-off eventually, but I'd hate to be interrupted in the middle of tracing some complex logic if there's a perfect stopping point 10 minutes later.
The world can wait. Fuck it. Just slack off and waste some time. That's what I do.
When my eyes feel bad, it's time to destroy an hour of the day, go for a walk and anyone who can't get on board with that can take a hike.
It's a very customizable tool - I've used it for months.
The more I think about it, the more of these issues that get solved naturally when working remote/at home.
Or managers (when in the office). :)
Joking aside, I view this as a necessary balance required in life. Even if my primary activity needs a lot of solo concentration, I must embrace the distactions as having value for reasons mentioned in the article. We are not ultra-specialized machines, even if we specialize in very narrow fields, we still live a life that comes with variety, diversity and in an environment filled with beneficial chaos.
Not quite what you're asking but it tracks how long you've been working (or using keyboard/mouse) and simply displays that without interrupting you at any point. If it's an hour or more for me, when I happen to glance at it, I get up and take a walk or stretch.
At school, I was taught that we have a short attention span of something like 10-15 minutes, so you could always do that. Every time the alarm goes off, go for a walk around the office.
Not quite what you were asking for, I know.