A heart rate of 38 beats per minute during sleep is lower than the textbook “normal” range, but it is not automatically dangerous. During deep sleep, your heart rate naturally drops well below its daytime baseline, and for some people, especially those who are physically fit, dipping into the upper 30s can be perfectly benign. The question of whether 38 BPM is a problem depends almost entirely on context: your fitness level, whether you have symptoms, what medications you take, and whether an underlying condition is driving the slowdown.
Why Your Heart Rate Falls During Sleep
Your heart does not beat at the same pace all night. As you fall asleep and transition into deeper stages of non-REM sleep, your nervous system shifts toward a state dominated by the parasympathetic branch, the part responsible for “rest and digest” functions. This shift slows your heart rate, lowers blood pressure, and reduces the workload on your cardiovascular system. A study examining heart rate variability across sleep stages confirmed that sleep onset and progression into deeper sleep were associated with greater parasympathetic activity, while REM sleep swung the balance back toward sympathetic (more activating) control.1PubMed Central. Circadian variation of heart rate variability across sleep stages Research comparing healthy sleepers and cardiac patients found the same pattern: the ratio of sympathetic to parasympathetic activity dropped significantly from wakefulness to non-REM sleep, then climbed back up during REM periods.2PubMed. Heart rate variability during specific sleep stages. A comparison of healthy subjects with patients after myocardial infarction
The practical meaning of this is straightforward. Your lowest heart rate of the entire day usually happens during deep non-REM sleep, somewhere in the first half of the night. A rate in the low 40s during this window is common even in people who are not athletes. A reading of 38 BPM captured during that phase is only a few beats below what many healthy sleepers experience. The real question is whether your rate is low because your body is doing exactly what it should, or because something else is pulling it down.
Athletic Conditioning and Low Sleeping Heart Rates
If you exercise regularly, your resting heart rate is likely lower than average, and your sleeping heart rate will be lower still. Endurance training enlarges the heart’s left ventricle, allowing it to pump more blood per beat. Because each beat delivers more, the heart simply does not need to beat as often. In collegiate cross-country runners, resting heart rates measured early in the season averaged around 44 BPM, with individual variation above and below that figure.3PubMed Central. Relationships between resting heart rate, heart rate variability and sleep characteristics among female collegiate cross-country athletes Those were daytime resting values. During sleep, with the added parasympathetic slowing described above, rates in well-trained athletes routinely fall into the mid-to-low 30s.
Professional cyclists, marathon runners, and other elite endurance athletes have been documented with sleeping heart rates in the 30s and even high 20s without any symptoms or adverse outcomes. If you are someone who runs, cycles, swims, or does other sustained aerobic exercise several times a week and you feel fine during the day, a sleeping heart rate of 38 BPM is almost certainly a sign of cardiovascular efficiency rather than disease.
When 38 BPM During Sleep Is a Concern
The dividing line between harmless and worrisome is mostly about symptoms. A heart rate of 38 BPM that causes no problems while you sleep, and that you only know about because a wristband told you, is very different from one accompanied by any of the following:
- Daytime fatigue or lightheadedness: If you feel exhausted despite apparently sleeping enough, or you wake up dizzy and it takes a long time to clear, your heart may not be meeting your body’s demands.
- Syncope: Fainting or near-fainting, particularly while lying down or shortly after waking, is a red flag that warrants prompt evaluation.
- Chest discomfort or shortness of breath: These can indicate that the slow rate is limiting how much oxygenated blood reaches your tissues.
- Long pauses between beats: Brief dips in rate are one thing, but if the heart actually pauses for several seconds at a time, the risk of passing out or worse goes up significantly.
In a published case, a 48-year-old woman presented after losing consciousness while lying down watching television. Monitoring revealed prolonged sinus arrest, meaning her heart temporarily stopped generating beats at all. The cause turned out to be moderate obstructive sleep apnea driving severe nocturnal bradycardia.4Journal of the Medical Association of Thailand. An Overweight Woman with Profound Symptomatic Nocturnal Bradycardia: A Case Report She needed a temporary pacemaker until the underlying sleep disorder was treated. That is the kind of scenario where a low sleeping heart rate is genuinely dangerous, and it is a world away from an asymptomatic dip to 38 picked up by a smartwatch.
Sleep Apnea and Nocturnal Bradycardia
Obstructive sleep apnea is one of the most common medical causes of a slow heart rate during sleep, and it deserves its own attention because it is frequently undiagnosed. When your airway collapses during an apnea episode, oxygen levels drop, and the body reflexively slows the heart. Research evaluating consecutive sleep apnea patients found that bradycardia occurred during 95% of all apnea events, whether they were central, obstructive, or mixed. The heart rate slowed by about 10 BPM during short apneas of 10 to 19 seconds, but the drop grew more severe with longer episodes: roughly 11 BPM for 20-to-39-second apneas and about 17 BPM for apneas lasting 40 to 59 seconds.5PubMed Central. Bradycardia during sleep apnea. Characteristics and mechanism
The mechanism is oxygen-driven. When researchers gave supplemental oxygen to sleep apnea patients, the bradycardia disappeared completely even though the apneas themselves continued and even lengthened.5PubMed Central. Bradycardia during sleep apnea. Characteristics and mechanism This tells us the slow heart rate is a reflex response to falling oxygen, not a structural problem with the heart itself. If you snore heavily, have been told you stop breathing at night, or wake up gasping, sleep apnea should be near the top of the list of things to investigate. Treating the apnea, usually with a CPAP machine, often resolves the bradycardia without any cardiac intervention.
Medications That Slow the Heart at Night
Several classes of prescription drugs can push your sleeping heart rate lower than it would otherwise be. Beta-blockers are the most common culprit, and many people take them for high blood pressure, heart failure, or after a heart attack without realizing they can cause significant nocturnal slowing. In a study of post-heart-attack patients on beta-blockers, bradycardia occurred in about 31% of them overall. The effect was strongly dose-dependent: roughly 12% of those on low doses developed bradycardia, compared with about 37% on moderate doses and 63% on high doses.6Medical Forum Monthly. Assessment of Beta-Blocker Induced Bradycardia in Post-Myocardial Infarction Patients: A Dose–Response Analysis Older patients were also more susceptible, with age over 65 roughly doubling the odds of developing bradycardia.6Medical Forum Monthly. Assessment of Beta-Blocker Induced Bradycardia in Post-Myocardial Infarction Patients: A Dose–Response Analysis
Calcium channel blockers like diltiazem and verapamil, certain antiarrhythmic drugs, and some psychiatric medications can also lower heart rate. If you recently started or increased a dose of one of these and your sleeping heart rate has dropped, mention it to your prescriber. A dose adjustment or a switch to a different medication within the same class can sometimes resolve the problem without sacrificing the drug’s benefits.
Thyroid Disease and Eating Disorders
An underactive thyroid is a well-known but sometimes overlooked cause of a slow heart rate. Thyroid hormones play a direct role in regulating how fast the heart beats. When levels fall, the heart slows down, and in severe cases, it can produce sinus arrest, where the heart’s natural pacemaker temporarily stops firing. A case report described a 53-year-old woman with hypothyroidism who experienced prolonged episodes of sinus arrest that resolved once she received thyroid hormone replacement.7Oxford Medical Case Reports. Unraveling the cardiac consequences of hypothyroidism: a case report of sinus arrest and bradycardia exacerbated by seasonal changes, escitalopram and medication noncompliance Most cardiovascular effects of hypothyroidism are reversible with proper treatment, which makes it an especially important cause to identify.
Eating disorders, particularly anorexia nervosa, can also produce strikingly low heart rates. Severe caloric restriction triggers the body to slow its metabolism as a survival mechanism, and the heart rate drops along with it. A meta-analysis found that people with acute anorexia nervosa had substantially lower heart rates than healthy controls, along with a pattern of nervous system activity dominated by the parasympathetic branch.8PubMed Central. Heart Rate Variability in Anorexia Nervosa: A Systematic Review and Meta-Analysis on the Moderating Role of Measurement Duration and Clinical Stage A sleeping heart rate of 38 BPM in someone who is significantly underweight or restricting food intake is not a sign of fitness; it is a medical emergency that reflects a body running on fumes.
REM Sleep Bradyarrhythmia
There is a rarer and more dramatic form of nocturnal bradycardia that occurs specifically during REM sleep. In this pattern, the heart does not just slow down; it can pause for several seconds at a time. A case report described a young man with no prior cardiovascular history whose heart rate dropped to 14 BPM during the night, with 412 pauses exceeding two seconds each and the longest single pause lasting 12.5 seconds.9PubMed Central. REM sleep-related bradyarrhythmia syndrome That is an extreme example, but it illustrates that the autonomic swings during REM sleep can, in susceptible individuals, produce alarming rhythms even in the absence of structural heart disease.
This condition is tricky because standard daytime heart monitoring often looks normal. The episodes only happen during certain sleep stages, so they tend to be caught on overnight sleep studies or extended multi-day heart monitors. If you have episodes of waking suddenly with your heart pounding, or if a bed partner has noticed you seem to stop breathing in an unusual pattern, this is worth discussing with a sleep specialist or cardiologist who can set up appropriate overnight monitoring.
What Your Wearable Is Actually Telling You
Most people discover their sleeping heart rate from a smartwatch or fitness tracker, and it is worth knowing the limitations of these devices. Consumer wearables use optical sensors on your wrist to estimate heart rate by measuring changes in blood flow through your skin. This technology, called photoplethysmography, works reasonably well for average heart rate over a few minutes, but it has documented weaknesses when it comes to finer measurements. A study comparing wearable readings to medical-grade recordings during sleep found discrepancies in the faster spectral components of heart rate variability, meaning the devices can miss or misrepresent the rapid beat-to-beat fluctuations that characterize certain rhythms.10PubMed Central. Heart Rate Variability from Wearable Photoplethysmography Systems: Implications in Sleep Studies at High Altitude
What this means for you is that a single reading of 38 BPM on your watch deserves context, not panic. The device may have caught a genuine dip during deep sleep, or it may have taken a slightly inaccurate sample during a brief movement artifact. If the reading is consistent night after night and lines up with how you feel, it is more meaningful than a one-off number. But a solitary low reading on a consumer device is not the same thing as a clinical finding on a medical-grade heart monitor. If you are genuinely worried, a Holter monitor or multi-day cardiac patch ordered by your doctor will give a much more reliable picture of what your heart is doing overnight.
Long-Term Outlook for People with Slow Heart Rates
One of the most reassuring pieces of evidence on this topic comes from a study that followed apparently healthy people over age 40 who had unexplained sinus bradycardia, meaning their hearts were slow and no one could find a clear reason why. Over the observation period, cardiac events like heart attacks, heart failure, and cardiac death occurred in about 8% of the slow-heart-rate group compared to 11% of the normal-heart-rate control group, a difference that was not statistically significant.11PubMed. Unexplained sinus bradycardia: clinical significance and long-term prognosis in apparently healthy persons older than 40 years In other words, having a slow heart rate without symptoms and without an identifiable cause did not increase the risk of heart trouble down the road.
This finding applies to people who were otherwise healthy and asymptomatic. It does not mean that all bradycardia is benign, but it does mean that a slow heart rate by itself, in the absence of symptoms or a known underlying disease, is not a ticking time bomb. For the person lying in bed staring at a watch that says 38 BPM and feeling perfectly fine, this is the most relevant data point.
Neurological Causes Worth Knowing About
Most discussions of nocturnal bradycardia focus on the heart and lungs, but the brain can also be responsible. Elevated intracranial pressure, the pressure inside the skull surrounding the brain, can trigger bradycardia through a reflex arc that connects the brainstem to the heart. A documented case involved a patient who developed recurrent profound bradycardia with sinus pauses during hospitalization. Draining a small amount of cerebrospinal fluid immediately and durably resolved the slow heart rate, confirming that the elevated pressure was the cause.12PubMed Central. The Heart-Brain Connection: Acute Intracranial Pressure Elevation as a Cause of Isolated Bradycardia The researchers emphasized that this kind of bradycardia can appear without the full set of classic warning signs of high intracranial pressure, making it easy to miss.
This is an uncommon cause, and it is unlikely to be the explanation for an otherwise healthy person with a slow sleeping heart rate. But it matters for anyone who has new-onset bradycardia along with headaches, vision changes, nausea, or a known neurological condition. In those cases, the heart may be a bystander, and fixing the brain problem fixes the heart rate.
Practical Steps If You See 38 BPM on Your Watch
If a wearable device shows a sleeping heart rate of 38 BPM and you feel well during the day, you have energy, you are not dizzy or fainting, and you do not snore excessively, the most reasonable next step is to simply keep an eye on the trend over several weeks. One night’s reading is not a diagnosis. Look for consistency. If your sleeping heart rate is typically in the low 40s and occasionally dips to 38 during what was probably deep sleep, that pattern is common in active people and generally benign.
If you are not particularly fit, if the number seems like a sudden change from your usual readings, or if you have any symptoms at all, schedule an appointment with your doctor. Mention the watch data, but expect that the clinical workup will involve more reliable tools: an electrocardiogram in the office, potentially a Holter monitor or multi-day patch, and blood work that includes thyroid function. If sleep apnea is suspected based on your history, a sleep study rounds out the picture. The goal is to distinguish between a healthy heart that beats efficiently and a heart that is being slowed by something that needs treatment. For most people who find this question worth searching, the answer ends up being the former.