Overnight Oximetry– The test involves applying a plastic clip over the end of your finger. Imagine a large clothespin enclosing your fingertip. This clip may be held in place with a piece of tape, but it is not painful to have on and it can be removed easily. It is connected via a cable to a small box that records the data overnight. Within the overnight oximeter clip is a red light. This red light shines through your finger. On the other side is a sensor that can measure your heart rate and the oxygen content of your blood. The latter is determined by the color of your blood, which will vary with the amount of oxygen. Highly oxygenated blood is more red, while blood that is poor in oxygen is more blue.