The bed is where Aeron is able to sleep, allowing you to change the time of day. This will cause Elena to be doing different things in different places. See Elena's Routine. This also allows Aeron to recover health. The longer you sleep, the more health he recovers. The bed is located in the living area under the landing on the second floor.
Elena's bed is next to Aeron's, and at certain times she will be sleeping or sitting on her bed. She will also go to sleep every time she eats some flesh. The bed cover can be changed to a different colour by giving Elena a drape, such as a red drape and a cat's-eye drape, and asking her to make a bed cover rather than a tablecloth.
The time will also affect certain aspects of the Towers. Time will pass in the Towers, so even if you enter at day you may come out at night. At night, the Towers are more dangerous. There are more enemies, and often they will be different or stronger than the ones you fight at day. At the same time, some enemies will be sleeping at night when you enter the room, as indicated by some bubbles above their heads. This gives you a chance to get in the first attack or to sneak past, although they will wake up if you move too close to them. The time will also affect which fountains you can drink from, restoring your health.