@djordje-s
Finally got response from dev team:
Please be advised that we need sufficient battery life for your watch to do this. Please follow the process bellow;
open Suuntolink > connect the watch to a computer
wait for the watch to sync with the Suuntolink
click gear sign and select watches
click the name of the watch 8-10 times.
check the box to put watch to deep sleep and then select reset watch.
After performing the factory reset please test the sleep tracking feature with the watch and let us know if the data that will sync to the Suunto application will be corrected.
The watch is showing correct sleeping time, problem is in app.
Only way is that watch is sync wrong data about the time frame.