Beds are objects in The Sims that increase a Sim's energy, comfort and decrease stress[TS3]. Both single and double beds can be found in buy mode under "Bedroom" or "Comfort". Cribs serve the same purpose for babies and toddlers. They can be found under "Kids". Single beds will only fit one Sim, while a double bed can fit two, but the two Sims will only sleep together if they have a good relationship. Kids will sleep together regardless.
Beds come in a range of prices from cheap to expensive. In general, more expensive beds do a better job of restoring energy and comfort. Also in The Sims 3 and The Sims 4, sleeping in cheap beds will give Sims negative moodlets when they wake up. In The Sims 2, Sims can "Relax" on a bed, and will often do so autonomously.
In The Sims Life Stories, there is a §9,000 bed that is only unlocked in story rewards for Riley. This bed is called the "Fertillinator", and will increase the chance of success when trying for a baby.
The Sims 3: Late Night added a new type of bed, the Vampiric Sanctum. This bed is in the shape of a coffin and can only be used by vampires. At one end of the bed there is a bottle of sunscreen lotion and a cracked mirror. There is also a picture of the The Magical Freezer Bunny on the front end and a moon on the bottom end. Vampires will receive the "Slept Like the Dead" moodlet from using this bed.
The Sims 3: Generations includes bunk beds, which can be used by kids and up. A few custom content creators for The Sims and The Sims 2 made working bunk beds for those games, but Generations is the first expansion pack to include them. Sometimes, Sims who wake up in the top bunk can be very daring and jump off.
In The Sims 3: Supernatural, children can check for monsters under a bed during nighttime. There's a 60% chance that they will see glowing red eyes or a monster's arms sticking out, which will give them "Monsters Under the Bed" moodlet, and possibly "Terrified" moodlet, which is more likely to be gained if Sims have Coward, Neurotic, Easily Impressed, and Supernatural Skeptic traits. While having the "Monsters Under the Bed" moodlet, children will not sleep on the bed the monster is from. Children will not get the moodlet if they have the "Someone to Watch Over Me" moodlet, which is gained by being promised protection by older Sims who are on the same lot.
On the other hand, if the children don't find any monsters under the bed, they will get "No Monsters" moodlet, which will boost their energy need while sleeping.