A long sleep for me is 7 hours, and I usually sleep 6.0 - 6.5 hours per night. I have scheduled 7 hours for sleep, but my watch still expects me to sleep 8 hours or so each night. I can see it now with the new sleep coach. This explains why Garmin always complains that I slept too short when I wake up well rested after a normal sleep. If I tried to sleep as long as the watch recommends, I would get more tired, and struggle to sleep the next night.
I can't seem to find anywhere to adjust the sleep goal. I can change step goals, so why not sleep goals? There are big differences between people. Some people need 5 hours, some need 10.