I've had a calathea for over a few months now and she's thriving at the moment. Being fairly new to house plants it's really an amazing feeling the plant actually survives. I observed that my Calathea's leaves are more upright at night. Three Google searches later I found Calathea's are part of the “prayer plant” (Marantaceae family) a type of plant that responses to light and dark. The leaves spread out to get more light and at night move upright to prevent moisture loss.
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From @masyl