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Forming a habit is a classic life hack: You want to change something about your life, so you insert a new action into your daily routine. Soon enough, it becomes automatic—that’s the dream, anyway—and then that part of your life takes care of itself. So how long does it take to form a habit? Despite what you may have heard, it’s not necessarily 21 days.

What happens in the first 21 days?

The idea that a habit takes 21 days to form came from a surgeon, Maxwell Maltz, who observed that it seemed to take about three weeks for a person to get used to their new body after an operation like amputation or plastic surgery, James Clear writes. (Having gone through a few minor surgeries, I recall that it takes between two and three weeks for pain and swelling to subside enough that you aren’t constantly thinking about the fact you just had surgery; I have to wonder if that’s related.)

The surgeon’s idea was that it takes 21 days for people to dissolve and re-form a “mental image” of themselves. This isn’t backed by research, it’s just one guy’s gut feeling. But I think it stuck because it fits a lot of our experiences.

Let’s say you want to get up early to exercise first thing in the morning. Well, of course you can do it once. But how do you make it a habit?

When I was in that situation—becoming a morning exerciser despite not being a morning person—I made myself commit for a full week before I even allowed myself to complain about it or adjust my plan. I had to be up at 6 a.m. every day, five days in a row, non-negotiable. I enjoyed having a break on the weekend, and then the second week was much easier. By the end of the third, this really was my new normal.

One day can be a fluke. One or two weeks is a timeframe we’ve been through before, and we can ride out a disruption that long (imagine a vacation, or a week’s crunch time before a work deadline). Three weeks, by contrast, is nearly a month. (Note that 30 days is also a common length of time for self-improvement challenges.) In this timeframe, you’ve done the thing a bunch of times, and you’ve likely weathered a few interruptions or obstacles (like the weekend) and gotten back on track. It’s probably a good rule of thumb for a timeframe that is long enough to feel like “real life.” But that doesn’t mean it’s enough.

What happens after 21 days?

There has been research that attempted to measure how long it takes for a habit to truly become automatic. For example, this study asked participants to choose a habit and to attach it to something they did once a day (for example, “eat a piece of fruit with lunch”). The study lasted 12 weeks. Some of the participants felt their new habit was automatic after just a few weeks; many others weren’t there yet at the end of the study. The researchers concluded that most people would form an automatic habit anywhere between two and eight months…according to a model that they calculated would only apply to 62% of the participants. Hardly a universal rule of human behavior, even with the wide range of possibilities.

A 2012 review looked at several other estimates and concluded that it would make more sense to tell people to expect at least 10 weeks for their new habit to become automatic, but also that it helps a lot just to know that any habit keeps getting easier the longer you do it.

How to think about behavior change in stages instead of days

Setting time-based commitments can be a helpful tool, like getting through the first week before changing the plan, or using your new moisturizer every day until the bottle is empty. But another school of thought holds that behavior change is better described by stages than by calendar dates.

In this idea, there are stages before you start the new action, like researching your options and giving the behavior a try before committing to making it a habit. But even after you begin the new behavior, the work continues. It’s not as simple as “do this forever.”

In the action stage, you’ve begun the habit, but it’s not automatic yet and you might not be convinced that you’re really going to continue. In this stage, you can do things like:

  • Remind yourself of your motivation to do it. For example, stick your reminder card for your next dentist’s appointment on your bathroom mirror, so that you remember not just that you should floss your teeth, but also why you want to floss your teeth.
  • Restructure your environment to give you cues and support. For example, if you want to run every morning, set your shoes out the night before and have your spouse ask you how your run went when you return.
  • Build self-efficacy by celebrating your small wins. This could mean checking off the days you did the thing on a calendar, but it could also involve working toward milestones (like total number of miles run) or making benchmarks of your progress (maybe you used to do your daily pushups with your hands on a chair, but now you can do them on the floor).
  • Plan ahead for how you’ll maintain your habit even when you’re interrupted (more about that in a minute).

Once you’ve built some momentum, you’re in the maintenance stage. You’re doing the habit, and maybe it’s starting to feel automatic, or at least more of a part of your life than it used to be. In this stage, you may need to do some things like the following:

  • Reevaluate your plan. Is running every morning still working for you? Maybe it makes more sense to make some of the runs longer and designate other days for rest, yoga, or strength training.
  • Think ahead to obstacles you might face. If you go on a vacation, will you continue the habit? If you end up falling off the wagon for whatever reason, how will you get back into it?
  • Make sure your motivation is something that will continue to work for you. For example, if you found it really motivating to keep up a streak on the calendar, the real test will come when you inevitably break your streak. At that point, there needs to be something other than the streak that is keeping you at it. This is often something intrinsic: You like being the person who flosses every day. You’re excited to sign up for a race with your running partner. You’re happy that your cholesterol is down from the way you’ve been eating.

Building a habit is not a matter of white-knuckling it until you hit a magic number of days. It’s a process that takes effort the whole time, even when you’re five years in. Habits are work, but the ones that last are the ones where the work feels worthwhile.