Recently, I have noticed that my Episcia (aka Flame Violet) often folds up its top pair of leaves when the night comes. The leaves are near the horizontal during the day and close to the vertical at night. And while sometimes the pair of leaves moves just slightly, pointing to the ceiling, there are times the leaves fold up so tightly, sticking against each other with hardly any gap in between.
As ignorant as I may sound, at first, I suspected that this movement of the leaves of my Episcia was due to insufficient sunlight (it had been rainy and gloomy for over a week). So, to keep my Episcia “open” at night, I put my plant under artificial light for around 14 hours per day for several days. However, the artificial light didn’t help at all. The top pair of leaves closes at night regardless of the amount of light there is during the day.
My baby Flame Violet, which I have propagated a while ago, also has its own circadian rhythms.