How does a repeated action gradually turn into a habit?

Answer: Repeating it in the same setting hands it to autopilot brain circuits

It locks in after exactly 21 days of repetition — There is no fixed 21-day switch; that number came from a 1960s self-help book. When researchers tracked volunteers building a new daily habit, it took anywhere from 18 to 254 days, depending on the person and the action.

Repeating it in the same setting hands it to autopilot brain circuits ✓ — Each time the same cue in the same setting leads to the same action and a small reward, circuits deep in the brain called the basal ganglia bundle the steps into one routine. Eventually the cue alone starts the routine, and the thinking part of the brain can step back.

Each repetition strengthens willpower until no effort is needed — Habits form by bypassing willpower, not by strengthening it. Once a routine runs on autopilot, the cue triggers it with little conscious effort, which is also why a bad habit persists after you decide to stop.