A low heart rate is called bradycardia. The term comes from Greek: “bradys” meaning slow and “kardia” meaning heart. In adults, bradycardia is defined as a resting heart rate below 60 beats per minute. But a slow heart rate isn’t automatically a problem. Whether it matters depends on how low it goes, what’s causing it, and whether your body is getting enough oxygen-rich blood.
When Bradycardia Is Normal
A resting heart rate between 40 and 60 beats per minute is common in healthy young adults and well-trained athletes. Very fit endurance athletes can have resting heart rates closer to 40 beats per minute, and this is a sign of cardiovascular efficiency, not disease. A stronger heart pumps more blood per beat, so it doesn’t need to beat as often to keep your body supplied.
Sleep also brings your heart rate well below the standard 60 bpm threshold. Your sleeping heart rate typically runs 20% to 30% lower than your daytime resting rate, putting most healthy adults in the range of 50 to 75 beats per minute overnight. During deep sleep specifically, your heart rate and blood pressure cycle down to their lowest points. Elite endurance athletes may see their heart rates drop into the 30s or even lower during sleep, which is generally harmless.
Symptoms That Signal a Problem
Bradycardia becomes a medical concern when the heart beats too slowly to pump enough oxygen-rich blood to the brain and other organs. When that happens, you may notice:
- Dizziness or lightheadedness
- Fainting or near-fainting
- Unusual fatigue, especially during physical activity
- Shortness of breath
- Chest pain
- Confusion or memory problems
Many people with bradycardia have no symptoms at all. The heart rate number alone doesn’t tell the whole story. A resting rate of 50 in a fit person who feels fine is very different from a resting rate of 50 in someone who keeps getting dizzy or blacking out. The Cleveland Clinic suggests seeking evaluation if your heart rate drops below 35 to 40 beats per minute, particularly if you’re also experiencing symptoms like chest pain, dizziness, or fainting.
What Causes Bradycardia
Your heart has a built-in electrical system that controls its rhythm. A small cluster of cells in the upper right chamber acts as a natural pacemaker, sending electrical signals that tell the heart when to beat. Bradycardia happens when something disrupts these signals, either at the source or along the pathway they travel through the heart.
One common cause is age-related wear on heart tissue. Over time, the cells that generate and conduct electrical signals can deteriorate, leading to a condition called sick sinus syndrome. In this condition, the heart’s natural pacemaker fires too slowly, pauses between beats, or alternates between abnormally slow and fast rhythms. Heart surgery, heart disease, and inflammatory conditions affecting the heart can also damage this electrical system.
Several underlying health conditions can slow the heart rate from outside the heart itself. An underactive thyroid is a well-known cause. When thyroid hormone levels drop too low, the heart contracts with less force and beats more slowly, often alongside symptoms like cold intolerance and fatigue. Electrolyte imbalances and obstructive sleep apnea can also contribute.
Medications That Lower Heart Rate
Some of the most common causes of bradycardia are medications people take for other conditions. Beta blockers, prescribed for high blood pressure, heart failure, and anxiety, work by deliberately slowing the heart rate and reducing its workload. Calcium channel blockers, another class of blood pressure medication, can have the same effect. Certain drugs used to treat irregular heart rhythms and some Alzheimer’s disease medications also list bradycardia as a side effect. If a medication is causing a problematically slow heart rate, adjusting the dose or switching to a different drug often resolves it.
How Heart Rate Standards Differ by Age
The 60 bpm threshold for bradycardia applies to adults and older children. Younger children and infants have naturally faster heart rates, so what counts as “too slow” changes with age. A newborn’s normal awake heart rate ranges from 85 to 205 beats per minute. For children aged 2 to 10, the normal awake range is 60 to 140. By age 10, children reach adult norms of 60 to 100 beats per minute. A heart rate that would be perfectly normal in a teenager could signal a serious problem in an infant.
How Bradycardia Is Treated
Treatment depends entirely on the cause and whether bradycardia is producing symptoms. If a medication is responsible, changing or stopping that medication may be all that’s needed. If an underlying condition like hypothyroidism is slowing the heart, treating that condition typically brings the heart rate back up.
When bradycardia stems from damage to the heart’s electrical system and can’t be corrected by addressing an underlying cause, a pacemaker is the standard long-term solution. A pacemaker is a small device implanted under the skin near the collarbone. It monitors the heart’s rhythm continuously and delivers tiny electrical impulses to keep the heart beating at an appropriate rate when it detects the rhythm dropping too low. Modern pacemakers are small, long-lasting, and allow most people to return to normal daily activities relatively quickly after implantation.
If you have no symptoms and your slow heart rate is explained by fitness level or normal sleep physiology, no treatment is needed. Bradycardia in these cases is simply your heart working efficiently.