There's tons of research showing THC impacts negatively REM sleep. A quick google search will return a lot of hits.

I doubt the quality of the studies. Also on my first hit “although this is not consistently replicated”.

Google is one thing, reading and interpreting the papers is another. What matters to me is that I feel great, regardless.

I think it's important to understand that there are trade-offs, even if this is a very powerful treatment. I tend to vary between THC-assisted sleep anywhere from 25% to 100% of nights and the trade-off of reduced REM is totally worth it. I do find that extended periods of 100% assisted sleep tend to make my memory a little foggy and that taking a week off resets things.

Caveat: anecdote is not data and all that...