Someone who walks in sleep is called sleep walker or somnambulist. Is there a word in English meaning somebody who talks in sleep?
There is somniloquent:
We are [referring to] sleep-talkers, a less-often encountered term than sleep-walkers, even though the former are more common.
Medical terminology has dignified words for them both: somnambulants and somniloquents.
Some sufferers have been known to do both at once: you might call this the Lady Macbeth syndrome.
As ambulant is primarily an adjective, and eloquent obviously is, doubtless the adjectival sense of somniloquent is in use. Indeed, English Word Information (of whom I've never heard till now) give only the adjective.
"Sleep talking, or somniloquy, is the act of speaking during sleep. It's a type of parasomnia -- an abnormal behavior that takes place during sleep. It's a very common occurrence and is not usually considered a medical problem.
The nighttime chatter may be harmless, or it could be graphic, even R rated. Sometimes, listeners find the content offensive or vulgar. Sleep talkers normally speak for no more than 30 seconds per episode, but some people sleep talk many times during a night.
The late-night diatribes may be exceptionally eloquent, or the words may be mumbled and hard to decipher. Sleep talking may involve simple sounds or long, involved speeches. " (Source)