The resting heart rate (RHR) is the number of times your heart beats per minute while awake and at rest. The nocturnal heart rate (NHR) is the lowest rate achieved during sleep, often detected by wearable devices. A heart rate of 38 beats per minute (bpm) is significantly low. While this number falls outside the typical range, it is not automatically a sign of poor health. Its significance is determined by the context of the reading, including your fitness level and any accompanying symptoms.
Understanding Normal Nocturnal Heart Rate
The heart rate naturally slows down during sleep as the body enters a state of deep rest and recovery. This physiological slowdown is driven by the parasympathetic nervous system, which promotes relaxation and energy conservation. For the average, healthy adult, the sleeping heart rate generally falls between 40 and 60 bpm, which is 10 to 30% lower than the daytime resting rate. Although a heart rate below 60 bpm is medically termed bradycardia, a nocturnal dip into the low 40s or even upper 30s is often considered benign for some individuals, especially highly conditioned athletes. The number 38 bpm must be evaluated against your individual baseline, not just the general population average.
Physiological Reasons for a Low Sleeping Heart Rate
Cardiovascular Efficiency
A heart rate of 38 bpm can be a normal and positive indicator of cardiovascular efficiency, especially in physically active people. High levels of cardiovascular fitness lead to “athlete’s heart,” where the heart muscle becomes larger and stronger. This enhanced strength results in an increased stroke volume, meaning the heart pumps a larger amount of blood with each beat. Consequently, the heart does not need to beat as frequently to meet the body’s needs, leading to a chronically lower resting rate.
Vagal Tone and Sleep
Endurance training also increases vagal tone, the influence of the vagus nerve and a major component of the parasympathetic system. This higher vagal tone keeps the heart rate suppressed at rest, and it peaks during the deepest phase of non-REM sleep, often yielding the lowest recorded heart rate.
External Factors
Medications are another common cause for a non-pathological low heart rate. Certain drugs, such as beta-blockers, are specifically prescribed to lower the heart rate and blood pressure, directly contributing to a low nocturnal reading. It is also important to consider the source of the data, as a poor fit or movement with consumer wearable devices could potentially lead to an inaccurate or falsely low reading.
Warning Signs and When to Seek Medical Attention
While a low heart rate can indicate excellent fitness, it is concerning if accompanied by symptoms suggesting the body is not receiving enough oxygenated blood. The presence of noticeable symptoms, not the number 38 bpm itself, is the most significant indicator of a problem.
You should seek a medical consultation if the low reading coincides with:
- Persistent fatigue or generalized weakness.
- Episodes of dizziness or lightheadedness, suggesting insufficient blood flow to the brain.
- Fainting or near-fainting episodes (syncope or pre-syncope), which require immediate medical evaluation.
- Shortness of breath, chest pain, or confusion and memory problems.
A physician will investigate non-benign causes, such as underlying heart rhythm disorders like sick sinus syndrome or heart block, which affect the heart’s electrical signaling system. Other potential causes include hypothyroidism or obstructive sleep apnea. If you experience severe symptoms like chest pain or fainting, seek emergency medical care immediately.