We currently do not have plans to add a custom "sleep goal" in Garmin Connect. We apologize for any inconvenience this may cause.
+1 to this problem
+1.
This seems like a no brainer to make sleep goal adjustable per the bed time and wake time and overcome the hard wired 8h goal. Go here and make the feature request.
This is discussed here as well:
https://forums.garmin.com/apps-software/mobile-apps-web/f/garmin-connect-web/136571/ability-to-change-sleep-goal-from-8-hours/1113995
Last input: share your ideas: https://www.garmin.com/en-US/forms/ideas/
Share your idea here:
https://www.garmin.com/en-US/forms/ideas/
This topic is discussed since 6 years. :-/
https://forums.garmin.com/apps-software/mobile-apps-web/f/garmin-connect-web/136571/ability-to-change-sleep-goal-from-8-hours/1113995#1113995