The Apple Watch can measure your blood oxygen level during sleep, but those readings can not be taken as a medical diagnosis. You should consult a doctor about this.
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Not a medical device Apple Watch is not a medical device and should not be used as a substitute for professional medical judgment. It is not designed or intended for use in the diagnosis of disease or other conditions, or in the cure, mitigation, treatment, or prevention of any condition or disease. Please consult your healthcare provider prior to making any decisions related to your health.
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copied from: Important safety information for Apple Watch - Apple Support