Why no REM tracking until I plug in and sync by Garmin Express?

Sleep interpretation is not 100% device driven. Garmin Express is not required. You can sync your wellness data either through your Connect Mobile APP or Garmin Express to see all details.

Yes, the problem is, as I described above, I don't get REM/advanced sleep tracking when using the Connect Mobile APP to sync. Once again today, I got no REM data and the same explanation as above that  my "device does not record REM sleep" yet, when I plugged in to Garmin Express, REM data immediately appeared. As you say, I should not have to sync with Garmin Express to get full sleep data, yet I do.

It looks like you have the watch associated with multiple accounts, and some data do not pass to the right account. It is a very common problem, and it usually happens when you reinstall the app (or sign out), and mistakenly sign up instead of signing in. Follow the instructions from the support document Data is Missing from My Garmin Account After Syncing to resolve the issue.

Checking the account info on the various ways to sync, I'm signed into the same account in all locations.

Once I sync in Express, the missing REM data shows up in the connect app. It just seems that the connect app does not upload complete data. It is the same account.

It still does not exclude that the device is associated with multiple accounts. You can be logged into the same account on GCM as on GE, but the device may be still linked to another account too. You need to break the association in the way described in the document I linked, if it is the case, and it is a quite credible possibility.

I only set up one account, but if I had another account and none of them were logged in on the Garmin Connect app, Garmin Express, or Garmin Connect Web, how could removing the device from those (as described) remove the device from the alternate account? I would need to remove the device from the alternate account, but if I can't find any place where that alternate account is logged in, how could I remove the device from that account? I think those instructions you referenced were created to avoid describing how to see what account is logged into in each of GE, GCM, and GCW.

It seems clear that my GE and GCM are using the same account, since what uploads and processes in GE then appears on my phone. GCM does upload and process sleep data but also tells me my device doesn't do REM, but I have an Instinct Solar, which does process REM. I did recently install an iPhone update and I wonder if the app just isn't working right since then. However, it seems many other people might be having the same problem in that case.

One thing I've noticed since working on this is that, while it takes an hour or so for my resting heart rate to update in the morning using GCM, it will update instantly and transfer right to GCM when I sync through GE.

4660547 said:

but I have an Instinct Solar, which does process REM

At Instinct Solar, the REM is being processed on the server, not on the watch. Currently, only Fenix 6 has recently a built-in sleep data processing. So if the sleep is not being processed when syncing over GCM, then it means either what I already wrote - that there are multiple accounts associated with the device, or that the syncing to the app does not work.

Following the instructions from the support document usually fixes the problem with multiple accounts, even if you are logged into the right accounts, and do not know the wrong one. The removing and the re-assigning the device to the right account does the trick.

For the case the syncing fails, browse through a few last Sync Audits  in GCM Menu » Settings » Device Sync Audit (at the very bottom). Use the arrow buttons to navigate through the history of the audits.

OK, I tried the procedure you linked, but to no avail. The same happened with last night's sleep. I realize some of the processing takes place on the server, but in the image above, "your device does not record REM sleep" is followed by an explanation of all the stages other than REM. If I sync through Express, that disappears and I get an explanation of all stages. REM also appears in Garmin Connect at that time. Obviously, the watch records what is necessary to get REM, it uploads data from overnight through Garmin Connect, but it apparently doesn't upload it properly since a Garmin Express sync causes REM data to process and appear. It seems to be an iPhone app issue. It was not like this until about a week ago.

It is entirely possible. Please contact Outdoor Product Support for your Instinct. We will gladly assist you further.