When there are no immediate tasks to do, why do you pause? What are you waiting for?
I assume you are waiting for something new to pop up. Maybe someone will need something from you, or you will remember an obligation you have to fulfill. Why then, are you waiting? Mindlessness is not a rest from your work—it’s a regression. You harm yourself by spending any amount of time being mindless. That is far different than resting, which serves a specific purpose and must be done so you can do your best work later.
What about resorting to entertainment? To distractions? To the approval of others? None of these things will give you what you are waiting for. Your remedy is simple—when you’ve done all there is to do, stop everything and reflect. Don’t stop working as a form of escapism. Use this time to reflect on the work you’ve done and completed and decide what your next step will be. Be clear, be direction-oriented, know your purpose, and act based on those things. I promise you that if you take time to plan how you will make progress, you will not think that any of your mindlessness is worth it.