Habits build well-worn synaptic pathways in our brains.

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Anyone who's ever tried to start an exercise routine, quit smoking, or change a sleep pattern knows how powerful a habit can be. Habits seem to be more than behaviors -- they seem to be part of who we are.

And in a way, habits are just that -- part of us. Habits are essentially patterns of behavior that become "worn in" to our brains. Someone who wakes up every morning, pours a cup of coffee and lights a cigarette, in that order, every morning, has that pattern built in to his or her brain, in the form of well-used synaptic pathways.

Everything we do (and think, for that matter) is governed by impulses firing across synapses, or spaces between certain cells that guide communication in the brain. When any behavior or pattern is repeated enough, the synaptic pathways associated with that pattern get used to being accessed. As a result, it becomes easier for impulses to travel along those pathways, and the behavior seems "natural." In other words, to the brain, wake-coffee-cigarette, in that order, is practically instinctive. One action triggers the next.

So when someone tells you, as many self-help gurus might, that you can form or break a habit in three weeks, it's natural to be skeptical. Why specifically three weeks? And how could you form a new instinctive behavior in such a short period of time?

In this article, we'll find out whether you really can turn a new behavior into a habitual one by repeating it for 21 days. We'll see where the belief originates and whether there's any hard evidence to back it up.

To understand what goes into forming or breaking a real habit, and how long that might take, it's helpful to look at what goes on in the brain once pattern-enforcing synaptic pathways are "worn in."