Issue related Sleep Mode

Hi

I have recently started using the sleep mode in my iphone to keep a track on my sleep record. It is working very well when system is picking up the sleep time as per my set schedules however, on some days I am sleeping early which is before my sleeping schedule and then I am turning my “Sleep mode” to ON via Control Center but when I wake up in the morning I have observed it never tracks the sleeping time duration when it was done manually.