In medicine, oxygen saturation (SaO2) measures the percentage of hemoglobin binding sites in the bloodstream occupied by oxygen. Maximal oxygen saturation is 100%. Normal waking oxygen saturation in a healthy child or adult is 96-99%. Normal asleep oxygen saturation in a healthy child or adult is 94-98%. Desaturation is a decrease in the percentage of oxygen found in the circulating blood supply. Oxygen desaturations <90% during sleep are considered medically significant. Patients with OSA may experience significant oxygen desaturations, with levels falling well below 90%.