No sleep detected

I do not know if I would call the ISM a "miserable piece of junk".  The ISM is very reliable for me and is performing as designed.  Sleep captured perfectly each night.  Sync's with my watch as it should.  No complaints all round to be honest.

Works pretty good for me too. I had two cases of missed sleep data over half a year though. However I am not sure if I can trust the data. Sometimes I wake up in the middle of the night and lye in bed thinking. For some reason Garmin does not register this as wake time.

So, it works for you. Congratulations! 
If it didn't work for the majority, Garmin would eventually care. But the number of people where it does NOT work is significant and Garmin obviously does not care. Never heard back about the ticket they opened and neither did anybody else. 

When I first got ISM, I had a few issues. Since the update shortly after launch, mine has worked perfectly and I love having it. 

There is no doubt that Garmin has launched this product as half baked & not paying attention to the issues...Congrats to those who r satisfied with the product...There is a sync issue if u r using multiple Garmin devices...I use 2 of the watches & to use ISM I hav to turn both my watches on Battery Saver mode otherwise it doesnt gives any sleep readings & on top of that my RHR & HRV readings r always haywire in ISM....It gives my RHR abt 10 points higher & my HRV mostly unbalanced....I hav never faced both these issues while wearing my Epix 2 & Fenix 8 watches....Always in luv with Garmin products but not this one....Garmin r u listeninggggg....!!!!...Pls. fix the issues...

The Index Sleep Monitor has been working perfectly for me for over a month, but today no data. I think the key for me is having another Garmin device. I'll experiment whether ensuring an Index sync has happened before putting my Garmin Fenix on helps.

For what it's worth I leave my FR265 turned but sitting on the table on at night, just not BT connected to GC.  The ISM is also not connected to GC at night.  In the morning I turn on my phone, take off the ISM and turn the ISM inside out, and the ISM syncs.  I leave the ISM alone and check in with GC on my phone several times while the sync happens.  As soon as I see the sleep graph in GC, I turn on BT on my watch and GC syncs with the FR265.  I let the watch sync for a minute or two before checking out the morning report.  All stats - sleep, body battery, etc - are all there in the morning report.

Hope that helps. 

Again - Congratulations. 
But no, this does not help others. Disabling BT or switching the watch off seems to improve percentage, but doesn't solve the problem 100%.
I returned my ISM and am using the Epix since then and always have perfect syncing. It'S just that bloody ISM that doesn't work.

I have two Garmin watches - Fenix 8 and Enduro 3. Still the Index works OK most of the time. I had two or three cases when I had no data during half a year period. There was another instance when the index died in the middle of the night. It showed no data, but after I charged the device it showed 4-5 hours of sleep before it went dead correctly. But I use iPhone for monitoring, which may be important.