Maybe you need a break after reading an intense project plan, or conversely, you need to take a break from working on a project to read the plan.
No matter the break's impetus, it ultimately comes down to having a distraction from what you were doing.
Consider taking a purposeful break -- one that isn't simply a distraction or escape from a previous activity, but, as the name implies, that has a purpose and therefore achieves a desired result. I find that doing so allows you to be more productive and to re-energize faster.
It's the same approach that we use for effective project meetings. Making sure that we focus on the agenda, follow all the topics and cover the intended elements. What works best in this case is staying focused on the task at hand, remembering the purpose and the planned or expected outcome.
To take a purposeful break, I suggest you do exactly what you want to do. For example, if you need five minutes to unwind after an intense meeting, do nothing else but listen to music. Don't try to figure out something about the project activity you were just involved in or what you are about to do next. Just sit quietly.
By allowing your mind to truly rest and disconnect, I find you are more effective at whatever activity you take on next.
When we focus on an activity completely, it reduces multitasking, and we are able to complete the activity in less time, at a higher quality and with a sense of accomplishment. It's contagious: the more you get done in less time, the more you feel you can do.
This information may seem like common sense, but taking purposeful breaks regularly is what is going to contribute to one's effectiveness in project execution and time management.