Ask HN: How to do intensive mental work without long chunks of free time?

Those of you who work on side projects whenever you find a short bit of free time, how do you do it? How can you do deep/intensive mental work when you do not have long chunks of time available without distractions?

I recently started doing two things:

- clocking in and out of some small tasks in my org files

- run a small app that will make a sound every three minutes. It reminds me to or be working towards my goals or take a short break (3 to 6 minutes) from too much work (which aligns with my goals).