I second "The Power of Habit", Duhigg is the kind of writer you're looking for - aggregating scientific sources and multiple studies, but presenting them in very easy-to-consume popular-science format.

Worth noting that the 21 day recommendation is "average", the actual "stick time" has a huge variation between a few days spent on "brush my teeth as soon as I wake up" habit to (likely) months that it would take to pick up the "run 5 miles before breakfast" habit.

Running five miles before breakfast is more than just a habit; it is also a skill. Many people simply can't run 5 miles right now, they need to first train up to acquire the ability to do it.

I agree with you, but I think the parent was saying that even controlling for one's ability to successfully do a thing, some things (i.e. those that are more strenuous or otherwise require more commitment) take longer to solidify as habits, which makes sense if we think of habit-forming as environmental adaptation. One doesn't adapt to massive environmental changes after just a couple of weeks.

Ironically, long distance running (10+ miles) is easier to do before breakfast, as an empty stomach is useful--digestion stops anyway as soon as your body is in need of some extra red cells to carry more oxygen. A trained person has enough ATP reserves for about 90 minutes at threshold intensity, and anyway the whole process of ingesting, digesting, and metabolising a regular meal takes many hours (around twelve in my case), so having breakfast before a run is purely a psychological boost--bar quickly metabolising food, such as banana or sport gels, which you do ingest during the run anyway.

All this of course depends on the individual's physiology and psychology.

For me, I tend to run either before breakfast or in the wee hours of the morning (one/two o'clock) before going to bed.

Mind, I'm an ultra runner, where being hungry and sleep deprived for days is the name of the game, so I've overcome that particular psychological barrier.

I should note that the book goes much deeper, offering insight into how habit-forming is used by marketers, how it impacts addiction, personal success, etc.

I second the recommendation for "The Power of Habit". I read it years ago and it has completely changed the way I think about human behavior and interpret the actions of the people in my life. Among the self-help books I've read it's probably the one book that has had the greatest impact on my life.

In addition, I haven't seen the third step in the "Cue -> Habit -> Reward" cycle mentioned yet. While the Reward may be the lingering endorphin rush from a hard workout or the feeling of accomplishment after finishing a chore, the author also suggests a simple, pleasurable behavior after the task that you're trying to make a habit of (like a small piece of chocolate immediately after a run or allowing yourself a few shameless minutes of cat videos after a focused study session). The idea is to allow your mind to associate the pleasure response with the habit each time it encounters the cue.

> I haven't seen the third step in the "Cue -> Habit -> Reward" cycle mentioned yet.

Good point! I allow myself an Irn-Bru only on run days (and less than 10 miles doesn't count). I don't live in Scotland and Irn-Bru is particularly hard to find, which is part of the incentive. Gives me a sense of accomplishment anyway!

The author talks of a variable reward. The variability is importance because of monotony associated with a repeated reward.

I'd also recommend reading the power of habit (I posted my own comment but then deleted when I saw this).

It talks through studies where people have broken bad habits through creating new ones.

+1 to reading The Power of Habit

Meditate. There are many different ways. Find one that works for you. It is the best way to become more conscious of your habits and less reactive.