A resting heart rate (RHR) is the number of times your heart beats per minute while you are awake and at rest, typically falling between 60 and 100 bpm. The sleeping heart rate (SHR) is a distinct measurement that naturally dips lower than the RHR. Wearable technology sometimes shows very low readings, prompting concern about a number like 39 bpm. This article clarifies the physiological reasons for a low sleeping heart rate and explains when 39 bpm should be considered a normal biological function or a potential medical concern.
Understanding Sleeping Heart Rate Physiology
The body undergoes a shift in control systems when transitioning from wakefulness into sleep, which directly influences heart function. During the day, the sympathetic nervous system (the “fight or flight” system) maintains a higher heart rate and blood pressure. As sleep begins, dominance switches to the parasympathetic nervous system (the “rest and digest” system), which actively slows down heart activity. This increase in parasympathetic activity is most pronounced during the deepest stages of non-rapid eye movement (NREM) sleep. The heart requires less work because the body’s overall metabolic demand decreases significantly during these deep sleep phases. This reduction in energy use and a slight drop in body temperature contribute to the slower heart rate. The heart is essentially resting and conserving energy, which is a normal and restorative biological process.
Contextualizing 39 BPM: Healthy Baseline or Bradycardia
For most healthy adults, a typical sleeping heart rate falls within a range of 40 to 60 bpm. Medically, a resting heart rate below 60 bpm is defined as bradycardia. However, this clinical definition is primarily applied to an awake, resting state, and exceptions are common for sleeping individuals. The 39 bpm reading, while technically below the 40 bpm lower end of the typical healthy range, is often considered a physiologically normal finding, particularly during deep sleep. This rate represents a healthy baseline where the heart is working efficiently to meet the body’s minimal needs. The distinction between a healthy low rate and a pathological one rests largely on the presence of symptoms. A slow rate that causes no adverse symptoms is usually not a cause for concern, even if it dips just below 40 bpm.
Non-Disease Factors Driving Low Sleeping Heart Rate
A number of external and physical factors, unrelated to disease, can push the sleeping heart rate toward or below 40 bpm. High cardiovascular fitness is one of the most common reasons for a naturally slow heart rate. Highly conditioned athletes often have resting heart rates well below the average because their heart muscle is stronger and pumps more blood with each beat, requiring fewer contractions. The specific stage of sleep also influences the heart rate, with the deepest stages of NREM sleep often showing the lowest readings. Certain medications can also exert a slowing effect on the heart rate, including beta-blockers and calcium channel blockers, which are commonly prescribed for blood pressure or other heart conditions. These factors can contribute to a consistently low reading of 39 bpm during the night.
Warning Signs Associated with Abnormally Low Heart Rates
While 39 bpm can be normal for a healthy person while sleeping, an abnormally low heart rate can be a sign that the heart is not effectively pumping oxygenated blood throughout the body. The most telling sign that a low heart rate is problematic is the presence of accompanying symptoms. These symptoms suggest that the brain and other organs are not receiving adequate blood flow. Symptoms that warrant immediate medical consultation include:
- Chronic fatigue and extreme weakness, especially when combined with the low heart rate reading.
- Dizziness, lightheadedness, or episodes of fainting (syncope).
- Shortness of breath or chest pain.
- Tiring easily during minimal exertion.
These symptoms suggest a potential underlying cardiac rhythm issue, such as Sick Sinus Syndrome or an Atrioventricular (AV) Block, which require professional evaluation.