From Nookipedia, the Animal Crossing wiki
Beds are furniture items in Animal Crossing series games. In Animal Crossing: Wild World and Animal Crossing: City Folk, a bed is located in the attic, where the player can save the game. This bed is same color as the roof of the player's house. In all games in the series, beds can be placed anywhere in the house and can be lied on, but the save prompt will appear only if the player uses the bed in the attic. The player can change the bed in the attic by switching it with another bed in the pockets.
Some beds in the games are double beds. When players lie on a double bed, they can move to roll over. In games that support Wi-Fi or local play, two players can lie together on a double bed. Several single beds and more can be placed together to act as double beds.
In Animal Crossing: New Leaf, beds are not required to save a game since the player can save anywhere in town, and players also start their game outside their house.
List of beds
In Animal Crossing: New Leaf, beds are no longer required to save the game. However, they may still be obtained and used as furniture.