Not all dwarves sleep at the same time, and dwarves don't sleep every day, but eventually all your dwarves will drop down. Sleeping dwarves will have their labor listed as sleep, and they will be labeled unconscious on the wound screen. Dwarves receive a happy thought from sleeping in a bed, and from choosing when they sleepVerify. In contrast, dwarves will receive an unhappy thought from sleeping on the floor, and from collapsing due to exhaustion.
Sleeping will occur in any constructed bed, this includes beds which have been designated as bedrooms, dormitories, barracks, and hospitals.
See Adventure mode for information on sleep as it pertains to adventurer mode