at rest

A malachite butterfly hangs from a leaf, upside down.

I've always wondered if there is something behind the direction a butterfly lights on a leaf. The American Museum of Natural History answers the question "do butterflies sleep?" with this statement:

At night, or when the day is cloudy, adult butterflies rest by hanging upside down from leaves or twigs, where they are hidden among the foliage. We don't really know whether or not they are sleeping.