Overnight Pulse Oximetry
Overnight pulse oximetry is a simple test worn on the finger during sleep that measures blood oxygen and heart rate, and the dips it records are a first-line screen for obstructive sleep apnoea.
Plain English
Patient-Friendly Explanation
Overnight pulse oximetry is one of the simplest tests in sleep medicine. A soft clip is placed on your finger and worn overnight; it shines light through the skin to measure how much oxygen your blood is carrying and records your heart rate at the same time. In a normal night, oxygen saturation stays steady at around 95% to 99%. In obstructive sleep apnoea the airway repeatedly narrows or closes, breathing pauses, and saturation falls by 3% or more - sometimes much more - before recovering with a brief arousal as you start breathing again. The number of these dips per hour is called the oxygen desaturation index, and it maps reasonably well onto how severe the apnoea is. The test is inexpensive, can be done at home, is well tolerated and is often used to decide who needs a full sleep study. Its limits matter as much as its strengths: it cannot tell obstructive apnoea from central apnoea, it can be falsely reassuring when breathing events cause arousals without much oxygen drop, and it can look falsely abnormal when your finger moves or when there is another lung problem. If you snore loudly, have been told that you stop breathing, wake unrefreshed, feel sleepy in the afternoon, or have blood pressure that will not settle, an overnight oximetry test is a sensible first step - and treating significant apnoea, usually with a CPAP machine, improves blood pressure, daytime energy and appetite regulation.
Medical
Clinical Definition
Overnight pulse oximetry records arterial oxyhaemoglobin saturation (SpO2) and pulse rate continuously and yields derived indices: the oxygen desaturation index (ODI, the number of 3% or 4% desaturations per hour of recording), minimum and mean saturation, and the proportion of the night spent below 90% (T90). Against polysomnography as the reference standard, ODI thresholds of roughly 5-10 events per hour give sensitivity in the region of 70-100% and specificity of about 60-90%, with better accuracy in moderate-to-severe obstructive sleep apnoea and poorer performance in mild disease; oximetry alone cannot distinguish obstructive from central events, cannot quantify hypopnoeas accompanied by arousal without desaturation, and is degraded by motion artefact and by coexisting cardiopulmonary disease such as COPD, obesity hypoventilation syndrome and heart failure. Guidelines therefore position oximetry as a screening and triage tool rather than a diagnostic study: a negative or equivocal trace in a patient with high clinical probability should proceed to home sleep apnoea testing or polysomnography, while a clearly abnormal trace with a suggestive history often justifies moving directly along the treatment pathway. Symptom questionnaires such as the Epworth Sleepiness Scale supplement interpretation. In obesity, oximetry carries additional information because nocturnal hypoxaemia and sleep fragmentation worsen insulin resistance, appetite regulation and blood pressure, and because obesity hypoventilation syndrome must be considered when baseline saturation is low or T90 prolonged. Parameters that should prompt escalation include an ODI above 15 per hour, minimum saturation below 85%, or T90 exceeding 10-20% of the night; response to therapy is assessed by repeat oximetry on positive airway pressure treatment, where residual ODI should be low and T90 minimal.
Key Insight
Why It Matters
If you snore loudly, wake unrefreshed or find yourself nodding off in the afternoon, ask about a home overnight oximetry test - it takes one night, costs little, and often settles the question of whether your sleep is breaking up dozens of times an hour. Untreated apnoea works against everything else we do: it worsens blood pressure, insulin resistance and appetite, and it makes weight loss harder.
Sources
Reviewed against standard medical references.
Medically reviewed by
Family Physician & Registered Acupuncturist, Accord Medical Clinic. MBBS (NUS), Graduate Diploma in Family Medicine (NUS), Graduate Diploma in Acupuncture (Singapore College of TCM).
Last reviewed: September 2026
This page is general health education, not medical advice. Whether any treatment is appropriate for you is a decision made in consultation after assessment.
