Yes it's OK to eat apples at night as long as you do not have any digestive issues.
Apples are OK to eat at night and they are a great late night snack and the apples are healthy for you as well.
But for some people the apples when eaten at night will sometimes produce gas and make you severely uncomfortable during the night especially if you suffer from Acid Reflux Disease.
But as long as you do not suffer from Acid Reflux Disease then you can eat the apples at night and then should be OK.
I love to cut up an apple at night and snack on the apples at night in bed while I browse the internet on my laptop and I actually find that I go to sleep better after having an Apple as snack at night.