Does missing one day break a habit?

What the study actually found

Ninety-six people chose one daily behaviour — a piece of fruit with lunch, a run before dinner — and logged it every day for twelve weeks. Automaticity was modelled for the 39 whose data fitted the curve. The median time to reach the plateau was 66 days, with a range from 18 to 254. So the honest answer to “how long does it take” is a shrug and a range.

Buried in that paper's results is the sentence people never quote: a single missed opportunity left the curve essentially unchanged. What did slow people down was missing repeatedly, early on — not the one Sunday.

A streak measures your record-keeping. It doesn't measure the habit.

Why the streak lies

A streak is a trailing count of consecutive days, so it can only ever do two things: climb by one, or fall to zero. That makes it a dramatic number and a bad one. Six walks in a week with one rest day reads as total failure to a streak counter, and as a good week to anyone else.

What to look at instead

Net is what you did minus what you said you'd do. It moves by one, in the direction you moved. Miss a day out of a five-day goal and Net goes from +1 to 0 — a small, true number that you can look at without flinching, which is the whole point.

So what do you do after a miss?

Nothing dramatic. Do the thing at the next opportunity. If the misses have started stacking up, the more useful question is whether the goal was right: a goal you set in an optimistic mood on a Monday is a guess, and a guess can be wrong. Lower it to what this month can actually hold, and the number starts telling you the truth again.