Battery drain in airplane mode

Hello to everyone, i am a new italian user..excuse for my bad english!
I have a problem with battery.
I own a series 0 apple watch 2015 (watchos 3.2) that if put in airplane mode it loses about 10% of battery in the night...Without touching the watch and with empty dock!
because?
it's normal?

It's a hardware problem

Am I the only one to happen to?