I run ads 24/7 unless there is a good reason not to do so. For example, a client does appliance repairs. He can't work 24/7 so he runs campaigns when he's actually open. He could have an answering machine or have people text him and get back when he can. But typically, when your fridge is on the fritz, or your furnace when it's ten below outside, you want someone there ASAP so there's no point in running ads all the time and waste money on clicks when you're in bed.
But a service where there is no rush, like mine where I manage PPC accounts, you ask to leave a message, email or whatever and you'll get back to them.
If you sell a product that can be bought online, why would you stop your ads in the middle of the night? There are people who are up at that time and do searches. Don't pass up the chance that one of them will buy. True, there is a lot less volume but you'd be surprised at how many people are up at those hours. Many search and buy.
Even if you are a brick and mortar store, don't pass up the chance to advertise. People know that stores aren't open 24/7. They are doing research. If they don't see your ad, you lose a chance of them knowing about you and visiting you. They'll go to a competitor instead.
