Battery draining overnight !!

I have 100 percent battery health . I have been using my brand new iPhone 12 mini for just 2 week . The battery drains on overnight . It does not drains the full battery . I turns on airplane mode when I go to sleep . Yesterday night there is 40% charge on my device when Iam going to sleep . After waking up morning the charge is 30% . The mobile data was already disabled and the phone is on airplane mode . My device is on the latest IOS 15.1.1