The 21-day myth
21 days is the most repeated number in self-improvement and it was never a finding. It comes from a 1960s surgeon who noticed patients took about three weeks to adjust to their new faces — an observation about grief, not habit formation.
The actual research came decades later: Lally and colleagues at University College London followed people building real habits and found automaticity took a median of 66 days — with a range from 18 to 254 days depending on the person and the difficulty of the behavior.
The shape matters more than the number. The steep part is at the beginning — which means the first two weeks are the most expensive ones you'll ever pay, and it gets cheaper from there. Missing a day early costs more than missing one at day 60.
Stop asking when it becomes automatic. Ask what today's square looks like.
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