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.
The T wave is the most labile wave in the ECG. T wave changes including low-amplitude T waves and abnormally inverted T waves may be the result of many cardiac and non-cardiac conditions. The normal T wave is usually in the same direction as the QRS except in the right precordial leads (see V2 below). Also, the normal T wave is asymmetric with the first half moving more slowly than the second half. In the normal ECG (see below) the T wave is always upright in leads I, II, V3-6, and always inverted in lead aVR. The other leads are variable depending on the direction of the QRS and the age of the patient.