Do you use WhatsApp personally? Or just for your business?
I would just create a canned response, and turn the app OFF after business hours. Turn it back on in the morning when you're expected to be at work, then resume.
Is the issue that it wakes you up at night? All phones have a method of disabling notifications form certain apps, and some can even turn off notifications during specific periods of time automatically. I'd suggest looking into those options.
The great thing about WhatsApp (from what I remember - It was a while ago!) is that it shows them when you received the message. But if you make it clear to them by communicating that you do not work after business hours, then they have no reason to get angry with you. Everyone needs downtime, and unless something has catastrophically crashed and burned, there's no reason to disturb someone outside of business hours.