The glass frog's translucent stomach skin offers a window into its internal organs, so much so that you can see its beating heart. This characteristic enables it to camouflage itself from predators: a common feature in marine animals such as ice fish or eel larvae. Scientists at Duke University have discovered that the transparency of Hyalinobatrachium fleischmanni, a species of glass frog, is doubled or tripled during sleep. Almost 90% of red blood cells are removed from the bloodstream and stored in the liver. When they wake up, they return the red blood cells to the bloodstream and the body becomes opaque. This knowledge could help prevent the formation of blood stones in humans.