Do you usually answer questions with a question? That's not how here works. It's abundantly clear they meant overnight. If they mean over 24 hours, guess what they would have said? You got it, 24hrs. To simplify it for you even more; during airplane mode, overnight, 3% of battery was used by iMessage. Think you have it now?
OP, I've had my battery stay at 100% overnight and at other times drop about 4%. It's very odd.