Perhaps you have noticed that soybeans look a little different as the sun goes down. It’s because they are sleeping.
Plants have their own internal clock which allows many internal metabolic processes to ebb and flow on a 24-hour cycle. These biological cycles are called circadian rhythms. It's this internal clock that allows pulses (including soybeans) to close their leaflets at night, a process called nyctinasty or “plant sleeping.” This phenomenon is not widespread across the plant kingdom and scientists are still unsure exactly why legumes and a few other plants do it. Though light and dark do play a role in this process, nyctinastic plants will continue the process of opening and closing their leaves on a 24-hour cycle even in an artificial environment of constant light.
Click to watch a bean plant fall asleep here, from Indiana University, Department of Biology:
So, when you see a field of soybeans looking a little droopy in the evening, just wish them good night. Their leaves will be back up first thing in the morning and working hard to produce a good yield.