Lying

Lying is a body position adopted by the player and some other mobs under certain circumstances.

Players
Players lie down when using a bed. Currently this is the only way that a player can adopt the lying position. Players cannot move, and cannot use items while doing so, unlike with sitting.

Cats
Cats will lie on beds whenever their owner is sleeping on one, and will themselves sleep. They emit specific sounds when doing so.

Foxes
Foxes will occasionally lie down and start sleeping during the day, taking shelter beneath blocks when doing so. Shade is a necessity for sleeping; if a path for direct sunlight is created that lights up a sleeping fox, it will immediately wake up and try to sleep elsewhere.

Pandas
Lazy pandas occasionally lie on their backs. Unlike with other mobs, this is a passive action, which is unrelated to sleeping.