A low resting heart rate has two broad categories of causes: physical fitness and medical problems. In healthy adults, a resting heart rate between 60 and 100 beats per minute is considered normal, though well-trained athletes routinely sit in the 40s or 50s without any issue. When heart rate drops below about 50 bpm and causes symptoms, the picture changes.
Fitness Is the Most Common Cause
Endurance exercise reshapes the heart in measurable ways. Years of running, cycling, swimming, or similar training cause the heart’s chambers to enlarge, allowing more blood to be pumped with each beat. The heart simply doesn’t need to beat as often to deliver the same amount of oxygen. Research published in Circulation found that the size of the right atrium, specifically, is an independent predictor of how low an athlete’s heart rate goes.
The mechanism goes beyond just a bigger heart. Training changes the heart’s internal pacemaker at a molecular level, downregulating the ion channels responsible for generating each electrical impulse. At the same time, the vagus nerve, which acts as the body’s braking system for heart rate, becomes more active. These two forces work together: a structurally remodeled pacemaker that fires more slowly, combined with stronger nervous system signals telling it to stay slow. Genetics also play a role. Some people are predisposed to lower heart rates, and endurance training amplifies that tendency.
During sleep, even non-athletes see their heart rate drop. A typical healthy adult’s sleeping heart rate falls to roughly 50 to 75 bpm, with the lowest points occurring during deep sleep. Athletes with high vagal tone can dip into the 30s while sleeping, which sounds alarming but is normal for that population.
Medications That Slow the Heart
Several common drug classes lower heart rate as either their intended effect or a side effect. If your resting heart rate has dropped recently, your medication list is one of the first places to look.
- Beta-blockers are among the most frequent culprits, prescribed for high blood pressure, heart failure, and anxiety. They block the stimulating signals that speed the heart up. Reported incidence of slow heart rate ranges from about 1% to 25% depending on the specific drug and dose.
- Calcium channel blockers like diltiazem and verapamil slow the electrical signals passing through the heart. Bradycardia occurs in roughly 4% to 16% of people taking diltiazem.
- Digoxin, used for heart failure and irregular rhythms, slows heart rate and can cause excessive slowing at higher blood levels.
- Certain antidepressants, particularly SSRIs like citalopram and escitalopram, can lower heart rate in a small percentage of users.
- Clonidine, a blood pressure medication, causes bradycardia in 5% to 17.5% of patients.
- Dementia medications like donepezil carry a surprisingly wide incidence of slow heart rate, reported at 0.6% to 48% depending on the study.
Even beta-blocker eye drops used for glaucoma can absorb into the bloodstream and slow the heart. If you’re taking any of these medications, a low resting heart rate is likely a pharmacological effect rather than a mystery.
Hypothyroidism and Metabolic Causes
The thyroid gland directly regulates how fast the heart beats. Thyroid hormones control pacemaker-related genes in heart cells and influence how those cells respond to adrenaline. When thyroid hormone levels drop, as in hypothyroidism, heart rate falls along with them. The result is a state of low cardiac output: slower heart rate, less blood pumped per beat, and reduced relaxation of blood vessels. This combination is why people with untreated hypothyroidism often feel sluggish, cold, and fatigued on top of having a slow pulse.
Electrolyte imbalances can also affect heart rate. High potassium levels (hyperkalemia) alter the electrical properties of heart cells, changing how quickly signals conduct through the heart. At moderately elevated levels, conduction can actually speed up briefly, but as potassium climbs higher, sodium channels become increasingly unavailable, slowing conduction dramatically. Severe hyperkalemia can make the heart electrically unresponsive. Low potassium creates a different set of problems, disrupting the ion pumps that keep cells electrically stable and causing abnormal calcium buildup inside heart cells.
Heart Conditions That Slow the Pulse
Sometimes the heart’s electrical system itself is the problem. The sinus node, the heart’s natural pacemaker, can degenerate with age or disease, a condition called sick sinus syndrome. When it fires too slowly or pauses for too long, heart rate drops. Similarly, heart block occurs when electrical signals are delayed or blocked as they travel from the upper chambers to the lower chambers. This can result from aging, prior heart attacks, infections, or inflammatory conditions.
These structural or electrical problems tend to develop gradually, often in older adults, and may go unnoticed until they cause symptoms. There is no single heart rate number that automatically requires a pacemaker. The decision depends entirely on whether the slow rate is causing problems.
When a Low Heart Rate Causes Symptoms
A slow heart rate that doesn’t cause symptoms generally doesn’t require treatment. Many people walk around with resting rates in the 40s and feel perfectly fine. The concern arises when the heart isn’t pumping enough blood to meet the body’s needs.
Symptoms of a problematically slow heart rate include dizziness or lightheadedness, fainting or near-fainting episodes, unusual fatigue that doesn’t match your activity level, shortness of breath with minimal exertion, and difficulty concentrating or a foggy feeling. In more severe cases, it can cause chest discomfort, confusion, or signs of heart failure like swelling in the legs.
The American Heart Association identifies several red flags that signal the slow rate is compromising circulation: low blood pressure, sudden changes in mental clarity, signs of shock, chest pain, or new heart failure symptoms. These situations require urgent evaluation. But a fit 35-year-old who notices a resting heart rate of 48 on their smartwatch and feels great is almost certainly looking at a sign of cardiovascular fitness, not disease.
Sorting Out the Cause
Context matters more than the number on its own. A resting heart rate of 45 means something very different in a competitive cyclist than in a sedentary 70-year-old who recently started a new blood pressure medication. The key questions are whether the low rate is new, whether it’s accompanied by symptoms, and whether there’s an obvious explanation like fitness level or medication use.
If your resting heart rate has always been on the low side and you exercise regularly, it’s likely a normal adaptation. If it has dropped noticeably without a clear reason, especially alongside fatigue, weight gain, or cold sensitivity, thyroid function is worth checking. If you’re on medications known to slow the heart, the timing of the change relative to starting or adjusting the dose usually makes the connection clear. And if you’re experiencing fainting, significant dizziness, or any of the more serious symptoms listed above, that warrants a prompt medical evaluation, often starting with an electrocardiogram to look at the heart’s electrical pattern in real time.