That is part of it but there is also something to be said about what is going on biochemically IMO. Even if you are feeling forward progress and comfortable about the scope of your problems, if you give yourself no time to rest and get out of a subconciously anxious state, that isn't very good.

Anxiety is meant to have your senses heightened to perhaps hear the tiger stalking you and encourage you to seek out a safer environment where you can comfortably rest. You aren't built to be in an anxious state for such extended periods of time. The tiger would have gotten you by then, with the way this system was designed. You aren't built to constantly run from the tiger.