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.
Image: Image: U waves are indicated by the arrows. In a normal heart beat, the T wave represents repolarisation of the ventricles, specifically the repolarisation of the AV node and bundle branches. The U wave occurs when the ECG machine picks up repolarisation of the Purkinje fibres.