Multitasking is a brain drain that exhausts the mind, zaps cognitive resources and, if left unchecked, condemns us to early mental decline and decreased sharpness. Chronic multitaskers also have increased levels of cortisol, the stress hormone, which can damage the memory region of the brain.
Each successful habit has the effect of saving cognitive budget for other parts of a person’s life. This is because a habit is something that doesn’t drain your cognitive budget (they are behaviors that originate in a different part of the brain). So any time we convert a habit from goal to routine, we’ve reduced a person’s daily cognitive load.
In fact, you don’t ever really delete old habits. You might stop using the neural pathway for your old habit, but the neurons are still there, waiting for a moment of weakness. Eventually those cells will die out. But there isn’t actually a way for you to train them to death.