I'd suggest soft-resetting the watch. Also make sure you run the latest firmware. Connect the watch over the USB cable to Garmin Express on a computer, and check for updates. Perform the soft-reset after finishing the update. If it does no help for the next night, try the master reset to factory defaults. If still not ok, contact the Support.
If you add image of the sleep phases showing the details then it is easier to help out. When start sleep and end sleep and the graph of the different sleep Stages. Also what are your sleep window set to for that day
