When Samsung S9 plus phone is in off mode the alarm clock will not work and neither the phone will get auto Switch on during the alarm clock is set on time. This feature are available with most of android phone,why Samsung is not adding it to s9 plus series and above phone.. This will save battery and also it's safe to put phone off while sleeping and not in use.
than you seduale your phone and fix the time. You choose your time when you sleep and awake and after you sleeped then phone switch off and before your arise phone automatically on. Eg (11:00pm turn off and 5am turn on)
it does not matter radio waves will still hit the phone whether it is switch off or on . one thing you can do it to keep it under airplane mode when not in use
