MIT researchers recently reported on a study that confirms that neurons in the striatum - which is found in the basal ganglia part of the brain and plays a major role in habit formation - fired at the beginning and end of a sequence of smaller habit steps, but did not need to do so, while the actual learned sequence was in progress.
Initially, when the habit was still very new, the neurons had to keep firing throughout the sequence, as the motions were learnt. Then over time, the same thing could happen, with the neurons only signalling the beginning and end - thus putting the intermediate actions on an autopilot. Something that happened, without effort.
We have to remember that when we consciously expend will power (read - neurons firing!) to do a new task, we do not have unlimited supply of will power. We can run out of will power half-way if we attempt too many new changes at the same time. We can be saved from this, distress, by letting smaller changes become habits that function on autopilot and then layering more habits on top of them.
On the other hand, if we kept putting variations in our newly learnt habit, then our neurons would tire themselves with the initial sequence itself, and we would not have too much will power to hold us steady on a bad day and for adding new good habits.
This is no different from first building the ground floor in house construction. Then effortlessly climbing the stairs from the ground floor, to go build the first floor and doing the construction there.
If you keep building and rebuilding the ground floor, for some strange reason, best known to you, then obviously not much work is likely to happen on the first floor and further up.
And yet, if you are anything like me - I start with intentions of a good habit and then tweak it every other day and then wonder why it has not settled into a routine!
If we let ourselves learn, what we are setting out to do as a new task and let it become an unchanging routine without variations, then it is easier to put it in place. Once this is in place, then more can be built on top of it.
This is key to designing big new habits, by layering what is new on top of what has now solidified.
Also see: http://news.mit.edu/2018/distinctive-brain-pattern-helps-habits-form-0208