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A lucid dream is a dream in which the dreamer realises that it's a dream, and can begin to control the dream. The term was coined by a Dutch psychologist in 1913, but scientific inquiry into this phenomenon did not seriously start until much later. In 1968, the writer Celia Green began to analyze the characteristics of such dreams. In the early 1980s, sleep researchers Keith Hearne and Stephen LaBerge independently discovered a method for lucid dreamers to signal their state to observers, proving their awareness during what's supposed to be a sleep phase. While the phenomenon has been established as real and is being studied as a valid field of sleep research, some New Agers have taken this as a starting point to make wild and unsubstantiated claims about the nature of consciousness, and that it might even be key to unlocking parapsychological powers.
People who believe that dreams have some sort of predictive nature are usually against any activity that prevents them happening naturally. Of course, dreams have no predictive ability beyond the normal amount of chance and shoehorning ability of the dreamer, so dream away - just seriously make sure you really are dreaming before you do something dangerous (a good nondangerous test is to see if you can hover/fly or phase your hand through objects). While this method has its uses as a stopgap measure until we've got a working holodeck and may even offer some of the benefits of meditation, it's important to note that there's no evidence whatsoever that you can use it to unlock any secret powers hidden in your subconsciousness. Flying and such may work while within the dream, but nobody should expect to gain special powers in real life.
 See also
- Lucidpedia - Would you like to make your dreams come true?