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Ok, I'm not sure if there is a trope for what I'm thinking of... If there was a name to give to it, I would call it "Bedtime Brainwashing".
The gist of the trope is when a character in a mentally vulnerable state similar to sleep (usually actually sleeping) when another character whispers in their ear in the attempt to subliminally influence their dreams or thoughts.
Type 1: The whispering character says something pleasant which makes the sleeper feel reassured or calmed, maybe even delighted.
Type 2: The whispering character attempts to trouble the sleeper's dream by plaguing his/her mind with troublesome thoughts and worries.
Type 3: The whispering character chants some sort of command or mantra so as to sublimially influence the sleeper's behavior when they wake up.
Example: Alice and Bob are sharing the same bed, and Bob wakes up when he overhears Alice's muttering in her sleep due to a bad dream, visably upset. Bob tries to whisper in her ear some sort of encouraging message or mantra for which to calm Alice down while she is completely unaware of Bob doing this.
Examples that come to mind. I can only think of a few in anime, but I'm sure there are other mediums that have them too...
Ebichu the Housekeeping Hamster: https://www.youtube.com/watch?v=oaAfGm6FHJk&feature=related
(3:15 to 4:15. NSFW)
Ebichu does both Type 1 and 2. He does Type 1 to her beloved master (normally depressed and desperate "Christmas Cake") who ends up waking up the next morning feeling completely refreshed and in good spirit.
Soon after, he tries to do Type 2 to said's master undesirable boyfriend by pretending to be the boyfriend's boss at work and calling him useless and all that. This attempt is subverted by the fact that he was actually awake the entire time and comedically beats Ebichu bloody.
History's Greatest Disciple Kenichi: http://www.mangareader.net/337-23554-11/historys-strongest-disciple-kenichi/chapter-48.html
Kenichi's martial artists masters use a Type 3 of this in the attempt to convince Kenichi to accept the Training from Hell they want to give him to prepare for an opponent he was having trouble with. The actual attempt doesn't work though.