Does sleep-talking typically happen during NREM or REM sleep? I have observed some of my mother's sleep-talking and have even attempted to reply / speak to her during such episodes.

In my experience, I have found sleep-talking to be a part of her dream. In some cases, when she sounded panicky, I have even woken her up partially to rescue her from panic, only to have her drowsily explain that she was dreaming.

However this article by Teodora Stoica in the Scientific American (https://blogs.scientificamerican.com/observations/why-do-we-sleep/) says that sleep-talking happens during NREM sleep and not during REM sleep when dreaming occurs:


  Onto stage 3 and 4, characterized by delta waves, the slowest and highest amplitude brain waves, the most unlike waking brain waves. Since delta sleep is the deepest sleep, it is the most difficult stage in which to wake sleepers, and when they are awakened, they are usually sleepy and disoriented. Interestingly, delta sleep is when sleep walking and sleep talking is most likely to occur.


Could the experts here please shed some light on sleep-talking and its relation (or lack thereof) to dreaming?
It seems like there is a common idea that dreaming occurs only during REM sleep, but I'm not quite sure why this persists. Scientists interested in sleep and dreaming have known for a long time that dreams occur in both NREM and REM sleep, and there are many papers comparing NREM to REM dreams.

Cavallero, C., Cicogna, P., Natale, V., Occhionero, M., &amp; Zito, A. (1992). Slow wave sleep dreaming. Sleep, 15(6), 562-566.



A fairly old paper on sleep talking concludes:


  The distribution of sleep-talking incidents by time of night does not differ significantly from a random distribution.
  
  Sleep talking occurs both in REM
  (rapid eye movement) and NREM (no rapid
  eye movement) stages of sleep.
  
  Sleep-talking incidents occurring during REM periods were different in character
  from those occurring in NREM periods


Of note, in looking at their data:


  Of the 84 sleep-talking incidents, 7 (8%) occurred in REM periods, 53 (63%) occurred in Stage 2, and 24 (29%) in Stages 3 and 4.


Therefore, it does appear that there are fewer events during REM sleep, but because REM sleep is a smaller fraction of the night this difference is exaggerated.

Related to dreaming,


  Only 2 awakenings were made following REM sleep-talking incidents. Both REM awakenings produced detailed reports of "disturbing" dreams.


and


  By any of the usual criteria, one would say
  that subjects are dreaming while speaking
  during REM periods. All of the recent data
  on the relationship between dreaming and
  REM periods indicate that dreams occur
  during these periods.


However:


  if we accept the NREM
  waking report of mental activity as repre
  sentative of the mental activity that was
  actually occurring during the incident, we
  would describe these mental experiences as
  less "dreamlike" than the REM experiences.


Therefore, it seems like sleep-talking during REM is more associated with dreaming; sleep-talking with NREM may not be, and may be more associated with internal thoughts or just brief arousals.

Rechtschaffen, A., Goodenough, D. R., &amp; Shapiro, A. (1962). Patterns of sleep talking. Archives of General Psychiatry, 7(6), 418-426.