Some of the most striking documented examples of flow come from people enduring extreme deprivation — solitary confinement, prison camps, isolated captivity — who kept their minds intact by deliberately constructing rule-bound mental activities out of almost nothing. This suggests flow isn't merely a pleasant enhancement to an already good life; under the right conditions, it functions as a genuine psychological survival mechanism, imposing order and purpose on a situation that offers neither externally.
What these cases share structurally is deliberate self-imposed structure: a goal, a set of rules, and a way of tracking progress, invented entirely from within because the environment provided none of it. The lesson generalizes past extreme circumstances — the same principle (imposing your own structure, goals, and feedback when the external environment gives you none) is available in far more ordinary situations of enforced waiting, isolation, or unstructured time.