The 21-day rule has a lot to do with automating , that is, every day we do things by inertia such as getting into the car and immediately fastening our seat belts. The same happens with this rule since automating an act causes the brain to involuntarily acquire it as a habit. Numerous studies show that this famous rule is how long it takes for human beings to adapt to a new change or habit . It is also compared to the time it takes for stem cells to differentiate into new neurons in the brain.

All living beings follow cycles that are related to nature and their own biological functions . Just as menstrual cycles last 28 days on average, there are other biorhythms such as emotional and intellectual that last less, to be exact those 21 days . This was concluded completely separately by researchers Michael Smolensky, an expert in chronobiology at the University of Texas (USA), and Zerrin Hodgkins, from London.

Therefore, if it is possible to maintain a behavior in a regulated and consevutive way during these three weeks , new routines can be achieved. You just have to follow the same pattern every day so that the brain adapts to the new situation. For example, when it comes to dieting, it is these 504 hours that it takes to assimilate this new change. On the contrary, after a couple of weeks if that habit is changed, the body will notice ‘something strange’ since little by little it has been adapting to the new routine habits.

In addition to establishing this rule , which has its benefits as proven by scientific studies around the world, we must also add the factor of willpower and mental balance because a change does not occur overnight. More than willpower, it can be said that to achieve the challenge you have to arm yourself with patience every day in order to remain calm throughout the process.


In 1887, the father of scientific psychology, William James , wrote an article, ‘The Habit’ , in which he exposed the enormous plasticity of the brain and how it takes 21 days for the formation of a new habit. Such was the reception of the theory that has reached the present day . On the other hand, Maxwell Maltz , a renowned plastic surgeon at Columbia University in the 1950s, and also known for endorsing the 21-day theory, began to notice a pattern that his patients followed: when he modified something A facial feature, for example, the nose, took them 21 days to get used to the new look.

Furthermore, he also observed that phantom limb syndrome in amputees followed the same pattern of the 21-day rule : “These and many other commonly observed phenomena tend to show that it takes a minimum of 21 days for a mental image to established disappears and a new one takes shape, “Maltz wrote in his book Psycho-Cybernetics (Psycho-Cybernetics: the secret to improving and transforming your life), a self-knowledge book that speaks of human potential, and that has sold millions of copies worldwide. world.

According to Maltz “we act and feel not according to reality, but to the image we have formed of it. Habits , good or bad, are formed in the same way.” For this to be possible, Maxwell demands that “all critical judgment and analytical reasoning be postponed during this period, in order to give himself an opportunity to check or deny the validity of these concepts for his life.”


Always living in the same way creates certain borders , fears and insecurities , so any change can generate some anxiety and uncertainty about what may happen. That is why while doing this challenge at all times, remain calm and have a certain motivation to face the new circumstances that are going to lie ahead. It is true that during the first weeks of the 21-day rule everything is uphill since the body and mind are being ‘taught’ other types of behaviors .

It must be borne in mind that each time an action is repeated it will cost a little less, because the neural path used is established more and more permanently until it eventually becomes an automatic act . Educating the brain to function as we want helps to achieve our goals. Once the 21-day rule is over, the anxiety will have disappeared, and we will enjoy a healthier and happier life and with a new habit in our life. Therefore, challenge achieved. 

Elle Mcdonald

I am Elle Mcdonald Specializations in Psychology . Graduated in psychology from the University of Tennessee in 2000. Diploma of Advanced Studies in the Department of Personality, Evaluation and psychological treatments with excellent results.

First Level of Master in Clinical Psychology at the Center for Behavioral Therapists (recognized with a scientific-professional nature by the College of Psychologists)

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