It might be a root problem. I had one earlier this year that was doing the same thing also even in the shade. It eventually died and when I pulled it out, I saw the roots were like barely existant when there should be lots of fibrous roots for a healthy plant.

I think it might have been stress from improper watering and caused most of the roots to rot away. I would let it go bone dry and then waterlogged it for days. Not good.

Cestrum nocturnum does like sun, but it's a water hog and a very fast grower. If it's a big plant that's been in its pot for 6 weeks or so, check on the water when you see it wilting, as it can outgrow it's pot that quickly.

That said, your pots look very large for the size of the plant, and the plants are new, so the comments by musaboru are well taken. You might have waterlogged soil because of the too big pot.

Keep it in the shade til you figure out what the problem is.

I've had a similar experience - mine wilt when the sun is at max intensity - noon - 2pm. But its been 2 weeks now and most of mine have adjusted. They do seem to like it when I water them around noon. The wilt actually goes away a few minutes after watering. So I'm guessing that its the water and heat combined - in my case. I should mention mine are in the ground though.

Try watering around noon, it might help.

Also, one other thing... if the pots are exposed to direct heat from the sun, the soil in the pots heats up more than the plant likes, and puts more stress on the plant. If its not getting direct sun, in the afternoon, check the area on which these pots sit, if the ground is hot, the pots will heat up too. If possible plant these in the ground, so that the soil temperature is more controlled and uniform.

All the best!

Mine does the same thing (in the ground). It wilts during mid day and if I water it after a few minutes it perks back up. If I don't manage to get around to watering it the leaves perk back up anyway as soon as the temperature cools a bit and the sun begins to set.

The ones in the picture look like they were cuttings taken and they might still not have a fully developed root system. Without a complete root system the plant is having difficulty keeping moisture retention in the leaves.

Take some soda bottles and cut the bottoms and cover the entire plant so that the leaves can stay moist, or mist them frequently throughout the day - either way should help. Do that for a couple of weeks so that roots can establish and they should be fine... all the cestrum cuttings I take tend to wilt and perk back up on and off, and they don't wilt at all if you cover them with plastic (I use bottles).