For years, this horrible app has been flubbing up recorded sleep data.
Immediately after syncing, my device will show (for example) that I slept from 11pm to 7am, with no fine-grained data like REM, or awake. The colored graph will be mostly featureless.
A few hours later, when I check back, many things change. For example, it will show new (incorrect) sleep and wake times, and the data will now show wakefulness and REM sleep.
This problem is consistent and happens everyday.
What watch are you using?
Watches up to Fenix 5+ still use the older "basic" sleep tracking, where the analysis is done on Garmin's servers based on the 24/7 data provided by the watch. That includes a quick analysis in Garmin…
That was my experience on my Fenix 5+ as well. Sometimes final data right away, sometimes with a delay. Perhaps it had to do with server load or whatever.
I just tried to glance through Garmin's pages…
Watches up to Fenix 5+ still use the older "basic" sleep tracking, where the analysis is done on Garmin's servers based on the 24/7 data provided by the watch. That includes a quick analysis in Garmin Connect right after sync, which is replaced by the final analysis after some time. And based on my experience (I used to have a Fenix 5+) the analysis is far from accurate, as it's only based on heart rate and basic movement data.
Newer watches (from Fenix 6 onwards) use "advanced" sleep tracking, which is done in the watch itself and also uses heart rate variability for sleep tracking. And still, I wouldn't trust the sleep phases of the newer sleep tracking either. It's just impossible to accurately detect from the wrist what happens inside your head. 
I synced last night at 10pm, after a full day, and it was the first sync for about 10 hours or more.
While syncing this morning when I woke, I took a screenshot of yesterday's sleep, then again immediately after syncing.
During sync:
After:
