The sleep tracking it's just a bad joke!

Every time I connect the watch to GC in the same day, the sleeping data is different and never accurate. On the first connection to GC it shows me for example 5 hours of sleep, but the second time I connect it to GC, right after gym training, it shows me 6-7 hours of sleep or more.
I agree that I have a strange sleeping pattern, working nights for 13 years, but Garmin's sleep tracking is just a bad joke.