Beds are furniture items.
The better quality of your bed, the more potential you'll gain after sleeping. Every town has an inn with beds (of subpar quality). Eastern beds are a great choice for early game, since they're lightweight and are fairly common to find, even at general goods store. Happy beds are a rare drop from cupids and fairies and seem to give the best stat potential when slept upon, and you can also get one from wishing. Alternatively, king's bed can be pickpocketed in Palmia.
It is advised to change the material type of any bed to a lighter material for easier carrying or more expensive material (e.g. diamondwork) for higher furniture and heirloom value to improve the home rank. Note that eastern beds don't seem to count as furniture, but are far better for portable sleeping arrangements than a sleeping bag. Also note that pets won't carry any beds save for eastern beds, sleeping bags and sleep sheep, since they won't carry any items which can increase the rank of your home.
Here's the complete list of beds. The gold value listed is how much it would cost if you had no negotiation skills.
|sleep sheep (E+)||1.3||2,100||140|
|Jure's body pillow||0.8||250||0|
The happy bed and double bed share the same sprite (#650 of item.bmp spritesheet) while the luxury bed uses a different located sprite (#262) with the same graphic.
The sleep sheep is exclusive in Elona+.
Also in Elona+, Jure's body pillow greatly increases the chances of Regeneration, Gaining Faith, Potential, and Development dreams at the cost of fewer bad dreams. It does NOT increase spell potentials.