I had this idea while reading La Berge's book:

The idea occurred to me that the most direct approach would be to use as a cue a sentence stating what the dreamer wishes to become aware of: \"This is a dream.\" I first tried this out in 1978 at the Stanford Sleep Laboratory, in collaboration with Dr. Lynn Nagel. Lynn got the short end of the deal, staying up all night monitoring my brain waves and REMs while I slept. When he observed me in REM sleep, he turned on a tape recording I had made earlier, playing it at a moderate level from a speaker next to my bed. The recorded message said, in my own voice, \"Stephen, you're dreaming\" and after a few seconds added the suggestion that I continue to sleep but realize that I was dreaming. At the time, I had not been sleeping too well, still being a newcomer to the sleep lab, and it seemed to me that I was lying in bed awake. Then from the next room, I heard the voice of a doctor commenting in Germanic accents, \"Amazing! Ze subject has had no REM sleep all night!\" Hearing this, I was not surprised. As far as I knew, I had had no sleep of any kind. But the next moment, I was astonished to hear my own voice coming over the PA. system, announcing, \"You're dreaming!\" I became lucid immediately. It had worked! I was very excited. In a dream world suddenly beautiful and more vivid than waking life, I was awake in my sleep! But a few seconds later, the recording continued with a voice now loud enough to wake the dead, to say nothing of the sleeping, \"Continue to sleep\"—and I awoke!
This first experiment showed us that lucid dreams could indeed be induced by direct verbal suggestion during REM sleep. The fact that in the dream I heard \"You're dreaming\" loud and clear, but did not hear my name at all, is interesting. Perhaps unconsciously hearing my name stimulated my attention, allowing me to hear the rest of the message consciously.
We used my own voice to record the message for two reasons. First, we hoped that being reminded by one's own voice would seem more like reminding oneself mentally, and second, because an earlier study found that when subjects heard tape recordings of their own voices during REM, the result was dreams in which the subjects were more active, assertive, and independent than when they heard recordings of other peoples' voices. Since these qualities are associated with lucid dreaming, we hoped that hearing my own voice would reinforce these qualities and facilitate the realization I was dreaming.
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You know how NovaDreamer works. It is the same principle. NovaDreamer flashes lights so you may see the effect in your dreams. And realize you are dreaming...

La Berge continues:

The results of this study also gave us a sense of the complexity and the multiplicity of variables involved. First there is the question of when is the best time to apply the stimulus, since not every moment of REM sleep seems equally suited to lucid dreaming. As for the message itself, what is its optimal form? First person—\"I'm dreaming\"? Second person—\"You're dreaming\"? Or objective—\"This is a dream\"? Our research at Stanford is aimed at finding our way out of this tangle of questions, to gain a reliable means of inducing lucid dreams in people who have had no prior experience with the phenomenon.
Another significant question is whether or not a verbal cue is best. In principle, any stimulus in any sensory mode could be used as a cue to remind a dreamer that he or she is dreaming. Perhaps a melody (say, Bach's Sleepers Awake!) might be more effective than speech. Or—since smell is the only sense that does not pass through the relay station in the brain called the thalamus, and thus may not be as inhibited as the other senses during sleep—it may be that scent would function as an especially effective cue.
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As i read through i came up with the idea to test this while i am wide awake.

I added a scheduled task that plays the voice file 2 hours later. And i started to watch a movie on TV. The movie was exciting. The movie was the dream and the wav file would remember me that i was just watching the movie.

I was fully concentrated on the movie and the sound file played. I jumped while lying on my bed. I thought "Whats happening? Who is talking? I am alone at home!" Then I remembered about the scheduled task as i realized the voice saying 'This is a dream.'

So i came to a conclusion. You have to be prepared about it first, otherwise it will wake you up. If i could remember about the scheduled task i wouldnt be scared and looked for someone in the room. As i concentrated on the movie i forgot about my life, my personality, my work and even my scheduled task. Just like in a dream!

So what do we have to do? We have to be prepared! Before going to bed i motivate myself to remember my mission. To realize that i am dreaming when i hear the triggering sound!
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