Lucid dreaming

The experience of being aware that one is dreaming in the midst of a dream.  Some lucid dreamers are able to control aspects of their dreams as well.  Lucid dreaming has long been a practice of Buddhist dream yoga.  Lucidity in dreams would help the student grasp that all experience is illusion.  Western science is only just recently begun to explore the mechanics and the applications of lucid dreaming, although the phenomenon has been described in historical and religious texts since the 5th century.  

Sources:

Dement, The Promise of Sleep

LaBerge and Rheingold, Exploring the World of Lucid Dreaming