A heart rate below 60 beats per minute is generally considered slow, a condition called bradycardia. But that number comes with a big caveat: population studies show that healthy adults can naturally have resting heart rates as low as 40 to 55 bpm depending on age and sex. A slow heart rate only becomes a medical concern when it causes symptoms or when your heart can’t pump enough blood to meet your body’s needs.
The causes range from completely harmless to potentially serious. Here’s what can make your heart rate drop and how to tell the difference.
Physical Fitness and Athletic Training
The most common reason for a low resting heart rate in otherwise healthy people is regular exercise. Up to 80% of endurance athletes develop bradycardia, and about 38% show minimum heart rates at or below 40 bpm on 24-hour monitoring. A small number, roughly 2%, even dip to 30 bpm or below during sleep.
This happens because sustained aerobic training physically remodels the heart’s natural pacemaker, the sinus node. Your heart becomes more efficient at pumping blood, so it doesn’t need to beat as often to deliver the same amount of oxygen. For a long time, this was attributed entirely to increased activity of the vagus nerve (the nerve that slows your heart), but newer research shows that the sinus node itself changes independently of nerve signals.
There’s also a genetic component. Certain inherited traits that influence resting heart rate appear more frequently in people who become endurance athletes. It may be that a naturally lower heart rate allows for greater cardiac filling, which leads to better cardiac output during exercise, essentially giving those individuals a built-in advantage. So your low heart rate might be partly something you trained and partly something you were born with.
Vagus Nerve Activation
The vagus nerve acts as your body’s brake pedal for heart rate. When it fires strongly, your heart slows down. This is a normal part of how your body regulates itself, especially during sleep, when vagus-driven slowing is the most common cause of a temporarily low heart rate.
Sometimes the vagus nerve overreacts. Standing up too quickly, straining during a bowel movement, bearing down, coughing hard, or experiencing sudden pain, fear, or emotional stress can all trigger what’s known as a vasovagal response. Your heart rate drops sharply, blood pressure falls, and you may feel lightheaded or faint. These episodes are usually brief and not dangerous, though the fainting itself can cause injury.
Medications That Slow the Heart
Many commonly prescribed drugs lower heart rate as either their intended effect or a side effect. The main culprits fall into a few categories:
- Beta-blockers (often prescribed for high blood pressure, anxiety, or heart conditions) directly suppress the sinus node’s natural rhythm.
- Calcium channel blockers like diltiazem and verapamil slow electrical conduction through the heart.
- Digoxin, used for heart failure and irregular rhythms, increases vagal tone and can push heart rate too low.
- Certain antidepressants, including citalopram and fluoxetine, can cause bradycardia by interfering with the electrical currents that pace the heart.
- Clonidine, a blood pressure medication, works by reducing the release of stress hormones that normally keep heart rate up.
Even beta-blocker eye drops prescribed for glaucoma can absorb into the bloodstream and slow heart rate. If you’ve recently started a new medication and notice your heart rate dropping, that’s worth discussing with your prescriber rather than stopping the drug on your own.
Thyroid Problems
An underactive thyroid gland (hypothyroidism) slows nearly every system in the body, including the heart. Cardiac output can decrease by 30% to 50% in hypothyroidism, and bradycardia is one of the most common cardiovascular signs. You’d typically also notice fatigue, cold intolerance, weight gain, and sluggishness. A simple blood test can confirm whether your thyroid is the issue, and treatment with thyroid hormone replacement generally corrects the problem, though heart rate may not increase dramatically even after other symptoms improve.
Electrical Conduction Problems
Your heart relies on a precise electrical system to coordinate each beat. When that wiring malfunctions, the signal from the upper chambers to the lower chambers gets delayed or blocked entirely. This is called heart block, and it comes in three degrees of severity.
In first-degree heart block, every electrical signal still reaches the lower chambers, just more slowly than normal. Most people never notice it, and it’s often found incidentally on an ECG. Second-degree heart block is where some signals get through and others don’t. In the milder form (Type I), the signal progressively slows until a beat is skipped. In the more concerning form (Type II), beats drop out unpredictably, making heart rate irregular and slower. Third-degree heart block is complete: no signals pass from the upper to lower chambers at all, and the lower chambers beat on their own at a much slower backup rate. This can seriously compromise blood flow.
Sick Sinus Syndrome
When the heart’s primary pacemaker, the sinus node, malfunctions on its own, doctors call it sick sinus syndrome. The heart rate may be inappropriately slow, or it may swing between too fast and too slow. There can also be sudden pauses lasting two to three seconds or more. Because symptoms come and go, a standard ECG often misses it. Doctors typically use a 24- to 48-hour heart monitor, or longer-term loop recorders for people whose episodes are infrequent.
Sick sinus syndrome can develop after heart surgery (especially in children who’ve had procedures on the upper chambers), in people with sleep apnea, or as a consequence of certain rare diseases like amyloidosis. It can also develop simply with aging, as the sinus node’s tissue degenerates over time.
Electrolyte Imbalances
Potassium plays a critical role in generating the electrical impulses that keep your heart beating. When blood potassium climbs too high, a condition called hyperkalemia, it disrupts the heart’s conduction system at multiple levels. At levels above 7.0 mmol/L, conduction through the area connecting the upper and lower chambers becomes impaired, and the heart’s backup pacemakers become unreliable. This can lead to heart block or, in extreme cases, the heart stopping entirely.
Hyperkalemia is most often caused by kidney disease, which impairs the body’s ability to clear excess potassium. Certain medications, including some blood pressure drugs and anti-inflammatory painkillers, can also raise potassium levels.
Sleep and Time of Day
Your heart rate naturally drops during sleep. This is driven by the vagus nerve becoming more active while you rest, and it’s completely normal. In young, fit people and trained athletes, nighttime heart rates in the low 30s or 40s with pauses of two seconds or more are common and require no treatment. If you’re seeing low numbers on a fitness tracker overnight, that alone isn’t cause for concern unless you’re waking up with symptoms like dizziness or unusual fatigue.
Warning Signs That Need Attention
A low heart rate by itself doesn’t always mean something is wrong. The key question is whether it’s causing problems. Symptoms that suggest your heart rate is too slow for your body’s needs include dizziness, lightheadedness, fainting, confusion or difficulty concentrating, shortness of breath, chest pain, and unusual fatigue.
If your heart rate drops below 40 bpm and that’s not your normal baseline, or if it falls into the 30s, your brain may not be getting enough oxygen. A heart rate in the 30s combined with any of the symptoms above warrants emergency care. The same applies if you experience chest pain, trouble breathing, or fainting alongside a slow pulse at any rate.