A puzzle left out overnight lets rest become another way of looking.

Stopping is not the opposite of working on something. Leaving a problem visible and going to sleep lets your attention arrive fresh, and the piece you stared past for an hour is often obvious in the morning. What changed was not the puzzle. What changed was the set of assumptions you had built up by grinding at it, which rest quietly dissolves.

When you next hit an hour of no progress, put the thing where you will see it tomorrow and stop. Treat that as a move you made rather than a failure to keep going.