In Dec 2014, I posted on WXC that I had a 45/min resting
heart rate (rhr). At the time, I just shedded 50+ lbs, was 
enjoying four runs a week, hadn't started weight-lifting,
and had coffee everyday. So my low heart rate, if it were a
problem, was not new and these years have been the best
for me health-wise.  Indeed I felt dizziness or craving for
sugar much less and less frequent than I used to.

Thursday morning, my rhr went down significantly after
breathing exercises. At one time I even recorded 40/min. I
didn't feel anything wrong besides a little lack of energy
due to poor sleep. That day, for some neck pain, I skipped
BJJ in the evening.

Friday morning, I took rhr upon waking up. 50/min. After
three rounds of breathing exercises, however, it dropped to
42/min. So the rhr drop I observed after breathing on Thu
was not a fluke.

Saturday morning, I woke up at 5:00am with a 50/min rhr. I
took Tim fishing, caught nothing, had a "tall" coffee at
Starbucks(after 2 weeks of no coffee), and came back home.
I measured the heart rate, 44/min. Wait! This was after all
morning activities and the coffee. What the hell?

I went on with my routine. After the breathing exercise, the
unthinkable happened: my heart rate dropped to a solid
38/min. I didn't know if this meant I had a super heart or a
super-frugal body, was going to die soon (too bad I have
just started to explore what health truly means), or
something else.

Here's what the Mayo Clinic says about normal heart rates:

    What's a normal resting heart rate?
        Answers from Edward R. Laskowski, M.D.

    A normal resting heart rate for adults ranges from 60 to
    100 beats a minute.

    Generally, a lower heart rate at rest implies more
    efficient heart function and better cardiovascular
    fitness. For example, a well-trained athlete might have
    a normal resting heart rate closer to 40 beats a minute.

    To measure your heart rate, simply check your pulse.
    Place your index and third fingers on your neck to the
    side of your windpipe. To check your pulse at your
    wrist, place two fingers between the bone and the tendon
    over your radial artery — which is located on the thumb
    side of your wrist.

    When you feel your pulse, count the number of beats in
    15 seconds. Multiply this number by 4 to calculate your
    beats a minute.

    Keep in mind that many factors can influence heart rate,
    including:

    - Activity level
    - Fitness level
    - Air temperature
    - Body position (standing up or lying down, for example)
    - Emotions
    - Body size
    - Medications

    Although there's a wide range of normal, an unusually
    high or low heart rate may indicate an underlying
    problem. Consult your doctor if your resting heart rate
    is consistently above 100 beats a minute (tachycardia)
    or if you're not a trained athlete and your resting
    heart rate is below 60 beats a minute (bradycardia) —
    especially if you have other signs or symptoms, such as
    fainting, dizziness or shortness of breath.

I don't know about the doctor's sample space when he gives
the normal range. He could have discounted outliers like me.
I do know how I feel (I feel fantastic, by the way) and read
about recent discoveries of the extremes of human
adaptation (Wim Hof comes to mind, who can control his own
heart rate). So my current strategy on heart rate, as it has
become part of my recovery game, is to keep paying
attention. While staying away from their drugs, I might even
consult professionals when it's convenient.

I learnt from the Web that the world record for low heart
rate was 27/min. I am not trying but I might accidentally
break it some day.