Obstructive sleep apnea by itself usually does not lower your oxygen level during the day. Most people with sleep apnea have normal daytime readings (about 95% to 100%) and drop only during sleep, when the airway repeatedly closes. If your oxygen is low while you are awake, it usually means something else is also going on, most often obesity hypoventilation syndrome, COPD or another lung disease (the “overlap syndrome”), or a heart problem. That combination needs prompt medical evaluation.
Key Takeaways
- In sleep apnea, oxygen drops happen during breathing events at night and recover when you wake up and breathe normally.
- Low oxygen while awake is not a typical feature of sleep apnea alone. It points to another condition that needs to be identified.
- Obesity hypoventilation syndrome (OHS) causes high carbon dioxide while awake and often low daytime oxygen. Many people with OHS also have sleep apnea.
- People with both COPD and sleep apnea tend to have lower daytime oxygen and deeper drops at night than people with sleep apnea alone.
- Sleep apnea can still make you feel as if you are short on oxygen during the day, through sleepiness, headaches and brain fog caused by poor sleep.
Why Sleep Apnea Mainly Lowers Oxygen at Night
In obstructive sleep apnea, the muscles that hold the throat open relax during sleep. The airway narrows or collapses, airflow drops or stops, and blood oxygen falls. After a few seconds the brain briefly wakes you, the airway reopens and oxygen recovers. This cycle can repeat many times an hour and produces the “sawtooth” pattern seen on overnight oximetry.
When you are awake, the muscles that hold the airway open stay active, so the obstruction does not happen. That is why a person can have severe sleep apnea and still read 96% on a fingertip oximeter at a doctor’s office. For details on night-time numbers, see what is a dangerously low oxygen level while sleeping.
When Sleep Apnea Comes With Low Daytime Oxygen
Obesity hypoventilation syndrome (OHS)
OHS is defined as obesity (BMI of 30 or higher) plus a high carbon dioxide level in the blood while awake (PaCO2 above 45 mmHg) that is not explained by another lung, nerve or muscle condition. Because the person is not moving enough air in and out, carbon dioxide rises and oxygen often falls, both day and night. Estimates cited by the American Thoracic Society suggest that roughly 8% to 20% of people with obesity and sleep apnea have OHS.
OHS is important to catch because it is more serious than sleep apnea alone, yet it responds well to treatment. The 2019 American Thoracic Society guideline recommends positive airway pressure therapy (CPAP, or noninvasive ventilation in some cases) and substantial weight loss. A simple blood test, serum bicarbonate, is used to help screen for it; an arterial blood gas confirms it. Learn more in our article on sleep apnea and high CO2 levels.
Overlap syndrome (COPD plus sleep apnea)
COPD can lower oxygen on its own. When COPD and obstructive sleep apnea occur together, people tend to have lower baseline oxygen and more severe drops at night than with either condition alone, and the overlap is linked to more cardiovascular problems and hospital visits. Research summarized by the American Thoracic Society notes that in COPD, nighttime drops are likely when daytime saturation is 93% or lower.
Heart and lung conditions
Heart failure, pulmonary hypertension, interstitial lung disease, pneumonia and other conditions can lower daytime oxygen. Some of these, such as heart failure, are also linked to sleep apnea, so the two problems can appear together. Long-standing, untreated low oxygen at night can also strain the right side of the heart. See can sleep apnea cause heart problems?
Why You Might Feel “Low on Oxygen” During the Day
Many people with sleep apnea describe feeling breathless, foggy or exhausted during the day even though their oxygen reading is normal. That is usually because repeated awakenings fragment sleep. Common daytime effects include:
- Excessive sleepiness and fatigue
- Morning headaches
- Trouble concentrating and memory lapses (see sleep apnea brain fog)
- Irritability or low mood
- Dry mouth or sore throat on waking
True shortness of breath at rest, chest pain or swelling of the legs are not typical of sleep apnea alone and need a medical check. See sleep apnea, chest pain and shortness of breath.
Checking Your Oxygen Day and Night
| Reading (awake, at rest) | What it generally means |
|---|---|
| 95–100% | Normal for most adults |
| 93–94% | Borderline; mention it to your doctor, especially with symptoms or lung disease |
| 92% or lower | Contact your health care provider (MedlinePlus, Cleveland Clinic) |
| 88% or lower | Seek immediate medical care |
People with COPD may have a different target range set by their doctor. A fingertip oximeter can be 2% to 4% off from your true level, and readings can be thrown off by cold hands, movement, nail polish and, as the FDA has warned, darker skin pigmentation.
To see what happens at night, an overnight pulse oximeter records your oxygen while you sleep and can show the repeated dips that suggest apnea. It cannot diagnose sleep apnea or OHS, but it gives your doctor useful data.
Treatment
- Sleep apnea alone: CPAP or APAP is the standard treatment; oral appliances and other options suit some people. Compare machines in our best CPAP machines guide.
- OHS: positive airway pressure (CPAP first-line for many people with OHS and severe OSA; noninvasive ventilation for others), plus weight loss.
- Overlap syndrome: CPAP for the apnea plus standard COPD treatment; some people also need oxygen.
- Daytime oxygen therapy is prescribed only when testing shows it is needed.
- Healthy habits help across the board: weight loss when needed, regular activity, not smoking, and avoiding alcohol and sedatives at night.
When to See a Doctor
Call 911 if you have severe shortness of breath, chest pain, confusion, or blue or gray lips or nails.
Make an appointment if your oxygen is 92% or lower at rest, if you have known sleep apnea and your daytime readings are dropping, if you have loud snoring with daytime sleepiness, or if your ankles are swelling. Ask whether you should be checked for obesity hypoventilation syndrome or COPD. Our guide on how to test for sleep apnea explains the next steps.
This article is for general information and is not a substitute for medical advice. Talk to a health care professional about your symptoms and test results.
Frequently Asked Questions
Can sleep apnea cause low oxygen levels during the day?
Usually not by itself. Obstructive sleep apnea lowers oxygen during sleep, and levels typically return to normal when you are awake. Low daytime oxygen in someone with sleep apnea suggests another condition, such as obesity hypoventilation syndrome, COPD or heart disease.
What is a normal daytime oxygen level if I have sleep apnea?
Most people with sleep apnea alone have normal daytime readings of about 95% to 100%. A resting reading of 92% or lower is a reason to contact your doctor, and 88% or lower needs immediate care.
What is obesity hypoventilation syndrome?
Obesity hypoventilation syndrome is a breathing disorder in which a person with obesity does not breathe deeply enough, so carbon dioxide builds up in the blood while awake (PaCO2 above 45 mmHg) and oxygen is often low. Many people with OHS also have obstructive sleep apnea. It is treated with positive airway pressure and weight loss.
Why do I feel short of breath during the day with sleep apnea?
Poor, fragmented sleep can cause fatigue that feels like breathlessness. However, real shortness of breath at rest is not typical of sleep apnea alone and may signal heart or lung disease or hypoventilation, so it should be checked by a doctor.
Will CPAP improve my daytime oxygen?
If your daytime oxygen is normal, CPAP will not change it much, but it prevents the night-time drops. If you have obesity hypoventilation syndrome, consistent positive airway pressure therapy can improve carbon dioxide and oxygen levels over time.
Sources
- Mokhlesi B, et al. Evaluation and Management of Obesity Hypoventilation Syndrome. Official American Thoracic Society Clinical Practice Guideline. 2019 (PubMed)
- American Thoracic Society — Sleep-Disordered Breathing in COPD
- National Heart, Lung, and Blood Institute — Sleep Apnea: Causes and Risk Factors
- MedlinePlus — Pulse Oximetry
- Cleveland Clinic — Blood Oxygen Level
- Sleep Foundation — What Are Normal Oxygen Levels During Sleep?
- U.S. Food and Drug Administration — Pulse Oximeters
