Try to wean off the night feeds, more than likely by now this is a habit over them actually needing it. However if you don’t want to do this and are happy to continue feeding in the night like above said your gonna have to go down to get it out the fridge so 40 secs in the microwave won’t make much difference to that x
I’ve started giving water and just comforting back to sleep when he wakes in the night.
I suppose you'd be going downstairs to get the milk from the fridge so warming it for a few seconds in the microwave won't make too much difference? Or just give it cold? My eldest and current baby didn't have a night feed at the cows milk stage so sorry I can't be of much help.