Blood oxygen saturation — SpO₂ — is the percentage of haemoglobin in your blood that is currently carrying oxygen. It is measured optically, and the method is elegant: oxygenated and deoxygenated haemoglobin absorb red and infrared light in different proportions. Shine both, compare what returns, and the ratio gives you the saturation.
What normal looks like
In healthy adults at sea level, resting saturation typically sits between 95 and 100 per cent. Brief dips across a night are ordinary — breathing changes with sleep stage and with position, and a single low reading in isolation is usually noise rather than news.
Altitude changes the picture entirely. So does a heavy cold. This is a measurement that only makes sense in context.
How Aurora reads it
Overnight only, when you are still and the signal is clean, and reported as a pattern across the whole night rather than as a single figure. A trend that holds for a week is worth attention. One number is not.
What it cannot tell you
This is the measurement most often oversold, so it is worth being blunt: Aurora cannot detect, diagnose or screen for sleep apnoea, or for any respiratory or circulatory condition. It is not a pulse oximeter in the clinical sense and it should never be used as one.
Optical saturation at the finger is also affected by how tightly the ring sits, how cold your hands are, how well perfused the tissue is, and by skin tone — a known and well-documented limitation of optical oximetry generally, not a quirk of this device.
If you are concerned about your breathing at night, that is a conversation to have with a clinician, and nothing on this page changes that.