You would want to watch Suni Williams' tour of the station videos:
It's the easiest to understand it. But in essence, they have sleeping stations like for example in the Node 2 (Harmony) module shown in video, with sleeping bags inside. If the video gets a bit confusing where some direction is, note that there's labels with orientations around the bulkhead behind Suni (OVHD for overhead, PORT,...).
You might wonder where the need to have those sleeping bags tied down comes from, if they're sleeping in microgravity. It's essentially to prevent them from unwittingly pushing themselves around during sleep, but there are slight external forces acting on them too, mainly air ventilation system preventing pockets of carbon dioxide, and slight gravity gradient that still exists in orbit and where the term microgravity comes from, describing its amplitude relative to 1 g. The bags are needed because the temperature at the station is kept slightly on the cold side of things if you're used to warmer climates (currently 23.20 °C in the Destiny module but the crew is up now with three out of six doing exercises) for various reasons, including the air itself serving as a coolant for various equipment, for hygienic reasons to prevent body odor, sweating, slow down growth of fungi and yeasts,... and due to air ventilation. And the doors are of course there to ensure some privacy.