Habit stacking assumes a stable life. Whose life is stable?

The advice to anchor a new habit to an existing one assumes your days have reliable structure to anchor to. That describes an office worker with a fixed commute. It does not describe a nurse on rotating shifts, a parent of a newborn, or anyone holding two jobs.

The book is honest that environment beats willpower, but the environments it uses as examples are unusually controllable. If your kitchen, schedule and commute are not yours to arrange, most of the system's leverage disappears.

So the open question is what replaces the anchor when there is no stable cue to attach to. Identity-based habits are the book's answer, but identity is the slowest lever it describes.

If your schedule changes week to week, what actually held? And what quietly stopped working the moment your routine broke?