It is, but I heard that it has a another synonym starting with "H"
Maybe this one happens when you have a fever or something and then you talk without control!
On a purely etymological note, I would suggest that the word you're looking for may be
hypnophasia. It's not a word I've ever come across, but it does the same job in Greek that
somniloquy does in Latin: 'sleep' + 'speech'.
b
PS I've checked in the OED online (subscription only), and they don't report 'hypnophasia' (or 'hypnoglossia' - another possibility that occurred to me, on the analogy of a few fairly technical and/or obcure words such as 'polyglossia' and 'heteroglossia') as being a word - not that existence in a dictionary is particularly significant... The day before publication of a dictionary that accepts 'new' words, do the words exist? (That was a rhetorical question, in case you hadn't noticed ;-); of course they do.)