Auto sleep tracking

Hey there, a quick question. In sleep tracking start options, the “bedtime” will not start unless I set an alarm? I select that option and now has been 2 days without the app tracking my sleep. I used to select it manually but sometimes I just fell asleep so I was hoping for the app starts automatically. How can I set this option, is there any custom start we can select? Because I can’t find it. Thanks for the help!