I guess this is a natural outcome of having the phone in your hand or on your body 24/7 and not being able to bear being separated from it even for a moment. :dubious:
See, where you see plague, I see godsend. It’s a free “do over”. What, I sent that? You know what, I must have dozed off and sleeptexted you. It is so a thing! I’ll send you a link to The Atlantic.
Kids today! Back in my day we just talked in our sleep, saying something like “The prince,the prince” much to the hilarity of our sleepover mates who thought we were envisioning marrying royalty.
Silly fools didn’t realize we were trying to comment on a book by Machiavelli.
I’ve met several people who sleep with their phone either next to them on the bed or with it under their pillow. I think they’re nuts. I certainly have mine on the night table, to use as my alarm clock, but it’s in silent mode and sometimes airplane mode while I’m sleeping. If I found out I was doing things with it in my sleep, I would banish it to another room and buy another alarm clock. Proper sleep is sacred to me.
The closest I’ve ever come to sleeptexting is reallyreallyreallyshitfacedmiddleofthenighttexting. They’re kinda the same, right? Both involve no next-day memory of said text. 
I predicted in the mid-1970’s (based on then-current technology) that by-and-by we’d have RS-232 sockets (a.k.a. COM ports) implanted in the sides of our heads, direct-wired into our brains.
Allowing for advances in the details of technology between then and now, I was basically right.
How about sleep-SDMB-posting? Now that I’d like to see.
Back in my very intense gaming days, I’d dream of being on the comp typing my fingers off. As it happened, sometimes I wasn’t in bed, I was sleeping at my desk and really typing.
When I talked about it between killing monsters, I learned that I wasn’t alone.
It doesn’t surprise me at all that people sleep text.
I can’t remember what to search for, but we have seen something similar–someone posting after their Ambien has taken effect.
I have once found a very incoherent message that I had sent during the night to a friend that I generally never correspond with, I just happened to have her number programmed in my mobile. Fortunately I had sent it to her landline telephone.
Back in 1999-2003 we had a couple guys in our Everquest guilds [The Haunnted Fold and Avengers Federation] that were notorious for falling asleep at their desk while still in game. One poor guy fell asleep with his toon dead and the reqpawn point bound in a spot he had thought was safe but really was in the pathing of a guard he was KOS with, so he died enough while he was asleep to lose 3 levels.
He had to get a GM to go and move his spawn point somewhere safe so he could get out of the death loop. 
I’ve thought basically the same thing. Cell phones are, in effect, a transitional technology on the road to the inevitable human hive mind.
You don’t read enough posts 
At least, that is what I tell myself. The alternative is too horrific to contemplate
I’m with you, si, I think sleep posting happens every day here.
zzzzzzzzz