I couldn't figure out why one of my plants looked parched/dry/thirsty even though I had watered the day before. Research tells me this plant is exhibiting a "nyctinastic" response to approaching night. The plant knew when it was day and when it was night, folded its leaves, and appeared to be napping. Wow, a nyctinastic response is fantastic. Do plants sleep? Yes, and they do a whole lot more. There are several types of nastic responses in the plant world.