by Gail Belsky. The term sleep disorder may suggest someone tossing and turning all night, but lying awake for hours with insomnia is just one example of many conditions that affect how you sleep and function during the day. In fact, you can have a sleep disorder and not even know it. Next: How long to snooze.
A U wave as seen on ECG. An electrocardiogram of an 18-year-old man showing U waves, most evident in lead V3. The U wave is a wave on an electrocardiogram that is not always seen. It is typically small, and, by definition, follows the T wave. U waves are thought to represent repolarization of the papillary muscles or Purkinje fibers.
ECG (EKG) (electrocardiogram): A record of the electrical impulses that immediately precede contraction of the heart muscle. The waves produced are known as the P,Q,R,S and T waves. An ECG is a useful means of diagnosing disorders of the heart, many of which produce deviations from normal electrical patterns. Electrocardiography causes no discomfort. Electrodes connected to a recording machine are applied to the chest, wrists, and ankles. The machine displays the electrical activity in the heart as a trace on a moving graph or on a screen. An ECG can be taken at home; a 24-hour record can be obtained from a tape recorder worn by the patient.