The term 'Upside-Down Sleeper' refers to a type of sleeping position in which the sleeper lies on their back with their head and shoulders propped up on a pillow. This position is often associated with two animals, the SLOTH (5 letters) and the BAT (3 letters). Sloths are known for their slow movements and their ability to hang upside down from trees for long periods of time. Bats are nocturnal mammals that sleep hanging upside down from branches or other surfaces. Both animals have adapted to sleeping in this upside-down position, making them ideal examples of the term 'Upside-Down Sleeper'.