noun
sleep talking; somniloquy
Refers to the act of talking while asleep, or the words spoken during sleep.
彼はよく寝言を言う。
He often talks in his sleep.
昨夜、隣の部屋から寝言が聞こえた。
Last night I heard sleep talking from the next room.
noun, noun which may take the genitive case particle 'no'
Figurative use: something as meaningless or absurd as words spoken in sleep. Often used dismissively.
そんな計画は寝言にもならない。
That plan is not even worth calling nonsense.
彼の言い訳はまるで寝言だ。
His excuse is complete gibberish.