There's little reason to turn your phone off, or even put it on airplane mode, overnight.
Just use Do Not Disturb; that's what the feature is for. You can tell it to turn on automatically around bedtime and turn off around the time you wake up, then you never have to worry about being bothered in the middle of the night.
You can also put in exceptions for favorite contacts if you want family members to be able to reach you in an emergency.