To sleep, you must first have a Premium Account. Next, and just as important, is access to a bed that is in a House or Guildhall. Finally, you simply need to "use" the bed and you will automatically be logged out while your character sleeps.
While sleeping, you will regenerate mana and hitpoints at a rate of four per minute, as long as the food you have eaten lasts.
Soul Points will be gained at a rate of one soul point every 15 minutes. It does not matter if you have killed a creature recently, you will always gain one soulpoint every 15 minutes while sleeping.
Food is also consumed at a different rate while sleeping. During normal play, if you are completely full, your food will last for 20 minutes. While sleeping, the same amount of food will last for 1 hour and 40 minutes (100 minutes).