Resting heart rate is the most responsive fitness marker most people have access to. It moves faster than VO₂max, it is measured every night without any effort, and it responds to things other than training in ways that are genuinely useful to know about.
What actually lowers it, ranked
1. Aerobic training volume
The dominant factor. Endurance training increases stroke volume, the amount of blood your heart moves per beat, so the same output requires fewer beats. This is a structural adaptation and it is why trained endurance athletes sit so far below the population range.
The important detail: easy volume drives this more than hard sessions do. The adaptations responsible for a lower resting rate come mostly from accumulated time at low intensity, which is another argument for not running your easy days too hard. Getting the easy zone right is zone 2 heart rate, and why yours is probably wrong.
Timeline: a few beats within four to eight weeks of consistent work. Larger changes take months.
2. Sleep
Fast-acting and frequently overlooked. Short and fragmented nights raise resting heart rate measurably, and correcting a chronic deficit can move it within a week or two. If your resting rate has crept up alongside a period of bad sleep, that is very likely the cause rather than lost fitness.
3. Alcohol
The largest single-night effect on this list. Regular drinking keeps your baseline several beats higher than it would otherwise be, and cutting it back produces one of the fastest visible improvements available. Detail in how long alcohol affects your HRV and sleep.
4. Hydration
Lower plasma volume means the heart must beat more often to maintain output. Chronic mild dehydration keeps resting rate elevated, and it is an easy thing to correct.
5. Psychological stress
Sustained stress keeps sympathetic activity elevated including overnight. This is real and not soft: a genuinely stressful few weeks at work will show in this number.
6. Body composition
Carrying less excess fat reduces the circulatory demand at rest. A slower effect than the others and a real one over longer periods.
Measuring it properly
Use the overnight figure rather than a spot check. A reading taken while sitting at your desk is contaminated by caffeine, posture, conversation and whatever you did in the previous hour. Your watch already records the clean version while you sleep, covered in what is a normal sleeping heart rate.
Then compare weekly averages rather than days. Night-to-night variation of several beats is entirely normal and reading individual mornings will mostly generate false conclusions in both directions.
When a lower number is not good news
A gradual decline alongside consistent training is adaptation. A sudden drop, or a low rate accompanied by dizziness, fainting, breathlessness or unusual fatigue, is a reason to see a doctor. Some medications, beta blockers in particular, lower resting heart rate directly and independently of fitness, which also means they make this metric unusable as a training marker.
Nothing here is medical advice, and this is one of the areas where the difference between an adaptation and a symptom genuinely requires someone who knows your history.
To raise aerobic fitness in general, which is the mechanism behind most of this, see how to increase your VO₂max.