Hi Liv and welcome to GoY, which plants are you talking about ones indoors, ones in the ground in the garden, ones in pots... lots more info needed please.

If in pots, it sounds like they are root bound. If this is the case remove them, trim back the root ball all around by 1/4 and replant in a bigger pot. If you prefer to use the same pot, trim back the root ball by 1/3. Use a very sharp knife such as a box cutter. After trimming and repotting it is normal to see some dieback of the plant as it resets itself to the root ball trimming. After a month or two it will start to fill out with new growth as new roots grow. This is one answer to one problem. As Moon Growe has asked, more detail will give you an answer more specific to your situation. No need to start a new question, just add that info to this current thread.

during the day the plant is using lots of water to make its food by photosynthesis, so by evening they are short of water and wilt. Over night they don't use water as they cant photosynthesise in the dark. But they are respiring and that produces water, just as we do. So they have more available water over night and this replenishes wilted stems.

then there are other factors such as water availability, wind speed, frost etc.

but which plants are we talking about though?

welcome to GoY :o)