Can Overnight SpO₂ Trends Detect Hidden Heart and Lung Disease? Understanding Pulmonary Hypertension and Heart Failure

Overnight SpO₂ trends can help detect hidden heart and lung disease by revealing patterns of low blood oxygen that may not be noticeable during the day. Conditions such as pulmonary hypertension and heart failure often cause recurring nighttime oxygen drops, making overnight SpO₂ monitoring a valuable screening tool. Modern wearables can now track these trends continuously, offering new opportunities for early detection. While wearables cannot diagnose these conditions on their own, persistent abnormal overnight oxygen patterns should prompt further medical evaluation.

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Why Night time SpO₂ Matters

Most people associate overnight oxygen monitoring with sleep apnea. However, researchers are increasingly discovering that the same overnight oxygen patterns can also reveal early signs of certain heart and lung diseases, sometimes years before symptoms become obvious. This is because many cardiopulmonary conditions interfere with the body's ability to maintain healthy oxygen levels during sleep, making the night an ideal time to detect subtle abnormalities.

In healthy adults, overnight blood oxygen saturation remains remarkably stable. The average nighttime SpO₂ is around 96%, with only small, temporary dips during certain stages of sleep. Brief drops into the low 90s can occur naturally, but frequent or prolonged oxygen levels below 90% are considered abnormal and may indicate an underlying medical condition. Even consistently sleeping below the mid-90s (for example, below 94%) should prompt further medical evaluation.¹

Several conditions can cause these nighttime oxygen drops. During sleep, breathing naturally becomes slower and shallower, particularly during REM sleep. While healthy lungs compensate for this without difficulty, diseases affecting the heart or lungs often cannot. Heart failure may allow fluid to accumulate in the lungs when lying flat, reducing oxygen exchange.² Lung conditions such as COPD or pulmonary fibrosis make it harder for oxygen to move into the bloodstream, an effect that becomes more noticeable during sleep.³ Repeated breathing interruptions caused by obstructive or central sleep apnea can also produce recurring oxygen desaturations. In other words, overnight SpO₂ doesn't point to one specific condition. It reflects how well the heart, lungs, and respiratory system are working together while you sleep.

To interpret overnight oxygen levels, clinicians look beyond a single reading and focus on overall trends. Common metrics include the mean SpO₂ (the average oxygen level throughout the night), T90 (the percentage of sleep spent below 90% oxygen saturation), and the Oxygen Desaturation Index (ODI), which measures how often oxygen levels drop by at least 3-4% each hour. Together, these measurements help clinicians determine whether overnight oxygen patterns warrant further investigation.

Can Overnight SpO₂ Trends Reveal Pulmonary Hypertension?

Pulmonary hypertension (PH) is a condition in which the blood vessels carrying blood from the heart to the lungs become abnormally narrow, stiff, or blocked. As a result, the right side of the heart has to work much harder to pump blood through the lungs. Over time, this extra strain can weaken the heart and eventually lead to right-sided heart failure if left untreated.

Early on, pulmonary hypertension is tricky to identify since its symptoms are typically subtle and easy to dismiss. Fatigue, breathlessness during exercise, dizziness, chest discomfort, and reduced stamina are commonly mistaken for aging, poor fitness, asthma, or stress. As a result, many people are diagnosed only after the disease has already progressed.

Researchers are now discovering that overnight blood oxygen monitoring may provide an earlier clue. Unlike daytime measurements, which often appear normal, sleep places additional demands on the heart and lungs. Breathing naturally becomes slower and shallower, making it harder for people with pulmonary hypertension to maintain healthy oxygen levels throughout the night.

Clinical studies have shown that nocturnal hypoxemia (low blood oxygen during sleep) is extremely common in pulmonary arterial hypertension (PAH), the most common form of pulmonary hypertension. In one study of patients with idiopathic and connective tissue disease-related PAH, 70% spent more than 10% of the night with an SpO₂ below 90%, while 87% of those patients spent over 20% of their sleep time below 90%.⁴ These findings suggest that significant overnight oxygen desaturation is the rule rather than the exception in many people living with PAH.

Even more importantly, researchers found that patients with nocturnal oxygen desaturation tended to have more advanced pulmonary arterial hypertension. Compared with patients without nocturnal desaturation, they had higher pulmonary artery pressures, greater pulmonary vascular resistance, lower cardiac output, and were more likely to exhibit right ventricular dilation, suggesting that overnight SpO₂ patterns may reflect disease severity in addition to identifying nocturnal hypoxemia.

Perhaps the most striking finding was that daytime oxygen measurements often failed to identify these patients. Around 60% of individuals whose oxygen levels appeared normal during the day still experienced significant oxygen desaturation while asleep. This highlights an important limitation of relying solely on daytime pulse oximetry and suggests that overnight monitoring may uncover problems that routine clinic measurements completely miss.

Why does this happen? Several mechanisms are likely involved. Narrowed blood vessels in the lungs reduce the efficiency of oxygen transfer into the bloodstream, and breathing naturally becomes shallower during sleep, making this problem more noticeable. Some people with pulmonary hypertension also have coexisting sleep apnea or chronic lung disease, which can further worsen nighttime oxygen levels. However, the fact that many patients continue to desaturate even without frequent apnea events suggests that the pulmonary vascular disease itself plays a major role.

What does this mean for wearable users? A smartwatch, smart ring, or overnight pulse oximeter cannot diagnose pulmonary hypertension. However, if it repeatedly shows long periods below 90% SpO₂, particularly without an obvious explanation such as diagnosed sleep apnea or chronic lung disease, it may indicate that further cardiopulmonary evaluation is warranted. In these situations, doctors may recommend additional tests such as echocardiography, pulmonary function testing, or referral to a pulmonary hypertension specialist.

Can Overnight SpO₂ Trends Reveal Heart Failure?

Although oxygen enters the body through the lungs, the heart is responsible for pumping oxygen-rich blood to every organ and tissue. When the heart becomes too weak or stiff to pump efficiently; a condition known as heart failure, blood can back up into the lungs, causing fluid buildup and making it harder for oxygen to move into the bloodstream. At the same time, reduced blood flow means less oxygen is delivered throughout the body. These problems often become more noticeable during sleep, making overnight SpO₂ an important marker of heart health.

Heart failure is also closely linked to abnormal breathing during sleep. Many patients develop Cheyne–Stokes respiration, a form of central sleep apnea in which breathing gradually becomes deeper and faster before slowing, followed by brief pauses in breathing. This condition affects approximately 25–40% of people with chronic heart failure and has been associated with poorer clinical outcomes.⁵ 

However, oxygen desaturation during sleep is not explained by sleep apnea alone. A 2024 study of 280 patients with heart failure and reduced ejection fraction (HFrEF) found that time spent with an SpO₂ below 90% (T90) was an independent predictor of five-year mortality. Every 1% increase in T90 was associated with an approximately 12% higher risk of death, even after accounting for age, cardiac function, and other clinical risk factors.⁶ 

Importantly, the researchers distinguished between oxygen desaturation caused by sleep apnea and oxygen desaturation occurring independently of apnea events. They found that non-apnea-related hypoxemia was the strongest independent predictor of mortality, suggesting that overnight SpO₂ monitoring may reveal worsening heart failure even in patients without severe sleep-disordered breathing.

A wearable device cannot diagnose heart failure. However, repeated overnight oxygen levels below 90%, a declining average nighttime SpO₂, or an increasing T90 over time may indicate that the heart and lungs are under greater stress than they should be. When these patterns occur consistently, particularly alongside symptoms such as breathlessness, fatigue, or swelling, they should prompt further medical evaluation, which may include echocardiography, blood tests, or assessment by a cardiologist.

How Wearables and AI Are Making Overnight SpO₂ Monitoring More Accessible

Until recently, tracking overnight blood oxygen required hospital equipment or dedicated sleep studies. Today, many smartwatches, smart rings, and wearable pulse oximeters can continuously record SpO₂ throughout the night, making long-term oxygen monitoring far more accessible.

Clinical studies have shown that modern wearables generally agree well with medical-grade pulse oximeters when oxygen levels are within the normal range, although accuracy may decrease at very low oxygen levels.⁷ Rather than focusing on a single night's reading, these devices are most valuable for identifying consistent trends over days or weeks.

Artificial intelligence is also beginning to improve how this data is interpreted. While today's algorithms are primarily designed to screen for sleep apnea, researchers are exploring whether AI can recognize oxygen patterns associated with conditions such as pulmonary hypertension and heart failure. As these models continue to evolve, wearables may become increasingly useful for identifying people who would benefit from further clinical testing.

If your wearable repeatedly shows average overnight SpO₂ below the mid-90s, prolonged periods below 90%, or frequent oxygen desaturations across multiple nights, it's worth discussing the findings with your healthcare provider. While these devices cannot diagnose heart or lung disease, they can help identify abnormal patterns early enough to prompt further evaluation.

What Should You Do If Your Wearable Shows Low Overnight SpO₂?

If your wearable shows concerning nocturnal hypoxemia, consider the following steps:

• Note symptoms: Pay attention to any related symptoms (fatigue, shortness of breath, swelling). If these occur along with low oxygen at night, mention them to your doctor.

• Consult your physician: Share your SpO₂ data or screenshots of your overnight reports with a doctor to review.

• Repeat in a clinical setting: A formal overnight pulse oximetry or sleep study in a medical setting can confirm the findings.

• Cardiac and pulmonary evaluation: If pulmonary hypertension or heart failure are suspected (based on your history or risk factors), ask about an echocardiogram, pulmonary function tests, or B-type natriuretic peptide (BNP) blood test.⁸ These can detect heart and lung dysfunction.

• Manage sleep-disordered breathing: If sleep apnea is diagnosed, treating it (e.g. with CPAP or BiPAP) may improve overnight oxygen levels and reduce strain on the heart and lungs.

Following these steps can determine whether an underlying cardiopulmonary condition is responsible for your low nighttime SpO₂, and guide appropriate treatment or monitoring.

Can Wearables Diagnose Heart Failure or Pulmonary Hypertension?

While overnight SpO₂ monitoring can reveal abnormal oxygen patterns, a low oxygen reading doesn't automatically mean you have pulmonary hypertension or heart failure. Several factors can temporarily lower blood oxygen levels, including high altitude, respiratory infections, chronic lung disease, obesity, poor sensor contact, cold hands, or even sleeping position. This is why doctors never rely on overnight oxygen readings alone. Instead, they interpret them alongside your symptoms, medical history, physical examination, and other diagnostic tests.

For the same reason, wearable devices should be viewed as screening tools rather than diagnostic devices. They can highlight patterns that deserve further investigation, but they cannot determine the underlying cause of those patterns on their own.

Some important limitations include:

• Wearables cannot identify the cause of oxygen drops. Repeated overnight desaturations could be caused by obstructive sleep apnea, pulmonary hypertension, heart failure, chronic lung disease, or several other conditions. Additional testing is needed to determine the cause.

• Consumer devices are not as accurate as medical-grade equipment. Most smartwatches and smart rings have an accuracy of approximately ±2–3%, and readings can be affected by movement, poor circulation, loose fitting, or improper sensor contact. They are most useful for identifying long-term trends rather than precise oxygen values.

• Many factors influence overnight SpO₂. Altitude, COPD, asthma, anemia, obesity, respiratory infections, and certain medications can all affect nighttime oxygen levels. These possibilities need to be considered before assuming an underlying heart condition.

• There is no single abnormal cutoff. While many studies consider measures such as T90 greater than 10% or an average overnight SpO₂ below 94% to be concerning, there is no universal threshold that diagnoses heart or lung disease. Clinical context always matters.

Conclusion

Overnight SpO₂ monitoring is emerging as a valuable tool for detecting more than just sleep apnea. Research shows that persistent nighttime oxygen desaturation may also be an early indicator of conditions such as pulmonary hypertension and heart failure. While wearable devices cannot diagnose these diseases, they can identify abnormal trends that might otherwise go unnoticed.

If your wearable consistently shows frequent oxygen drops, prolonged periods below 90%, or an unusually low average overnight SpO₂, it's worth discussing the results with your healthcare provider. Used alongside clinical evaluation, overnight SpO₂ monitoring can help identify potential heart and lung problems earlier, when further investigation and treatment may have the greatest impact.

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References:

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7. Jiang Y, Spies C, Roghanizad AR, Wang WK, Bhosai SJ, Snyder L, Burke A, MacLeod D, Dunn J. Performance of Wearable Pulse Oximetry During Controlled Hypoxia Induction: Instrument Validation Study. JMIR Form Res. 2026 Mar 27;10:e85253. doi: 10.2196/85253. PMID: 41894520; PMCID: PMC13026435. 

8. B-type natriuretic peptide (BNP) test: Normal levels & purpose. (n.d.-b). https://my.clevelandclinic.org/health/diagnostics/22629-b-type-natriuretic-peptide