I set the Goodnight Mode on my Galaxy Watch. After 3 (three) days, it began setting automatically based on my Daily Routine/Habits. The Goodnight Mode is available on my watch by swiping down from the top of the watch and pressing the Moon Icon. Hope this helps.
Another option would be to go to your Samsung Health app > tap on the sleep option > tap the 3 dots in the top right corner > tap set target and it will allow you to choose the start and end times for your sleep
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