Battery backup

Hello. I am using iphone SE 2020, my phone’s battery drains A LOT in standby (overnight) & in battery section it doesnt show any usage!! Same thing happened last night, i charged it to 95%, turned off the wifi and reduced brightness and slept. When I woke up the phone was too hot and battery percentage was 48% ??

my phones battery health is 94%

as for today’s reduction in battery i wonder maybe its because i put my phone under the pillow. But why does it always happen??