How long does it take to form a habit?

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As you’re drafting your New Year’s resolutions, you may think that it takes 21 days of repeating an action for that action to become a habit. So, you set out to go to the gym for 21 days, thinking that by day 22 heading to the gym will feel automatic — maybe even fun. It can be daunting to think about going to the gym for a whole year, but 21 days is doable.

We hate to burst your bubble, but that 21-day estimate isn’t true. According to habit expert and myth buster Wendy Wood, this falsehood came from a self-help book in the 1960s and actually described how long it takes to get used to your new appearance after plastic surgery.

How long does it take for a habit to form? It’s a question many of us want answered in those early, effortful days of habit-building. When will I floss every morning without having to think about it? When will I no longer need a reminder to take my medication? When will choosing to hit the gym feel easy?

Unfortunately, our recent research shows that no magic number exists.

So what are we supposed to do? We know that people with well-established habits need to rely less on willpower to execute good behaviors, but the early days of executing a good behavior typically feel like a slog for everyone. Only after consistent repetition will the desired behavior start to feel more effortless.

We did find a few actionable, science-based tips that might help you get there faster.

We used machine learning, which is a type of artificial intelligence, to analyze data on tens of thousands of gym-goers and hospital workers in North America in the hopes of better understanding the way two important habits form: habits around exercise and habits around handwashing.

We defined a habit as the point at which a behavior becomes highly predictable for a given person using our statistical modeling tools.

Here’s what we learned that could help you:

We would all love to believe exercising or doing another difficult new activity will feel automatic in three short weeks. Instead, we found hints in our research that the speed of habit formation may be correlated with the complexity of the habit we’re trying to form.

Take handwashing. Although everyone is different, people typically form handwashing habits in one to two weeks, while forming a gym-going habit typically takes months. In our study, we only analyzed the formation of two types of habits, but we suspect that simple habits such as handwashing or toothbrushing may become habitual even faster than the old 21-day myth would have suggested.

Why does it take months rather than weeks for gym-going to become habitual? We only compared two habits in our study, but our guess is that the complexity of going to the gym slows habit formation down. Going to the gym takes significant time, effort and planning. And it’s at most a daily habit, making it far less frequently executed than, say, handwashing. In general, past research suggests that more repetitions are key to building habits.

What does this mean for you? If you’re in the process of starting a “Couch to 5K” training plan, don’t beat yourself up if you aren’t putting on those running shoes without thinking by week four. It will likely take a few months for your workouts to feel automatic. A good dose of patience will serve you well.

If you want to build a habit around physical activity faster, consider trying a quicker form of exercise — say, doing a few jumping jacks or squats — and doing it hourly. Doing so might help put your new habit on autopilot in a shorter time frame.

It is common to think that forming healthy habits depends on willpower, but habit researchers (including our team) do not recommend “willing” yourself to create a habit. Instead, we encourage you to focus on creating habit-friendly situations — environments that will eventually “cue” a desired behavior.

Cues can include elements of your physical environment, the time of day, specific objects or even people you encounter. For example, seeing the digital clock in your bathroom flash “6:30 a.m.” may cue you to brush your teeth if you have consistently brushed your teeth at this time in the past.

Thinking through when and where you plan to follow through on a goal is critically important to successful habit formation, past research suggests. We’ve also seen good results with the use of quirky cues to trigger people’s memories. So, if you want to start going to the gym, our and other people’s research suggests it’s best to plan the day of the week you want to go and maybe add a unique cue such as an alarm on your phone that plays “Physical” by Olivia Newton-John whenever you’re meant to hit the gym.

But don’t despair if you don’t find the right cues right away. In the early days of building any habit, you should anticipate an exploratory phase since parts of whatever you’re trying to put on autopilot will be novel and you’ll need to learn how to incorporate them into your daily life.

It’s worth it. We found that the longer we observe someone, the more predictable patterns of exercise (or handwashing) become. After a period of experimentation and repetition, you can eventually establish predictable habits.

The dark side of habits is that once they’re formed, you enact them reflexively — even if that’s not the best option. If you were consciously and effortfully choosing fruit over a muffin every morning, you would probably notice if your apple looked rotten. But if you’re grabbing an apple on autopilot, you might discover it’s bad only after you’ve walked out the door and taken your first bite on the way to work. Yuck!

Is this risk real? Do we respond less to information suggesting we should or shouldn’t be sticking with an activity or choice after it’s become habituated?

We were able to test this idea in our data by looking at how gym-goers respond to various interventions incentivizing exercise, which make it temporarily more rewarding to hit the gym. When splitting gym-goers into those who our algorithm determined had formed a habit and those who had not, we explored whether these incentives mattered more for one group than the other. As we anticipated, we found people who had already formed a habit were less sensitive to new rewards for gym attendance.

What does this mean for you? Habits will make you less flexible when there’s a valid reason to change your behavior. In fact, some variability in a routine can build more durable habits. So be careful about which behaviors you execute frequently and make sure you want those things to become habitual. Behaviors that we may think of as “bad habits,” such as incessantly checking your phone, are just as susceptible to becoming automatic and harder to change.

In January, as you set resolutions and try to put them on autopilot, remember that you can’t count on habits to kick in after a magic number of days. If it has been 21 days and you still have to put a workout on the calendar to make it a reality, don’t lose hope. There’s nothing wrong with you, and a gym-going habit may still be around the corner.

Habits aren’t a pipe dream. With repetition, most people can eventually develop predictable routines that are hard to knock off course.

Colin Camerer and Hung Ho contributed to this article.