Interior scenes in daylight are often lit by the light that enters the room through windows or open doors. This light has traditionally been popular with artists because of its constancy and its simplifying effect.
Assuming the sun is not shining directly through the window, the daylight that enters a room from outside is a soft source, meaning the light comes from a broad area and it doesn't cast hard-edged shadows. In this group portrait by Peder Krøyer (1851-1909), note how the shadow cast by the vase quickly dissolves into blurry edges.
When window or doorway light falls on a floor, there's often a lot of fall-off, or weakening of the light, as you travel back from the window, as with this interior scene by Anders Zorn (1860-1920). This is because the amount of window light available is usually in proportion to how large a slice of the blue sky you can see from a given point in the room.
Even though Zorn used a very limited palette, he loved to play with the relationship between the slightly cool window light and the relatively warm interior light from incandescent or flame-based lamps.
Thanks, Tim A. And thanks to BoingBoing for picking up the studio shot.
See a bunch more high-res images by Krøyer at Tim's blog.