The Oxygen Desaturation Index (ODI) is the number of times per hour of sleep that the blood's oxygen level drops by 3 percent or more from baseline. ODI is typically measured as part of standard sleep studies, such as a polysomnogram or overnight oximetry. These drops in oxygen levels are called desaturations. ODI is measured by an oximeter, which is a device typically placed on the fingertip that shines a red light through the skin and can estimate the amount of oxygen in the peripheral blood. When breathing becomes disrupted during sleep, as may occur in obstructive sleep apnea, the oxygen levels of the blood may repeatedly fall. These drops are typically associated with collapses of the upper airway, events called apnea or hypopnea. Drops occur less frequently in snoring or upper airway resistance syndrome (UARS), two conditions in which breathing is disturbed, yet to a lesser degree. ODI may be worsened in people with underlying lung disease, and an elevation in ODI may lead to increased oxidative stress in the body that may predispose people to long-term cardiovascular risks, including heart attack, stroke, and memory loss associated with dementia. These consequences are an active area of sleep research.