Ambient Occlusion

The floor, walls, and ceiling are Arch & Design mats ...

... which have Ambient Occlusion off:

In this version, we turn Ambient Occlusion on for those objects:

15 minute render:

Please not that below the mat, the lamps, etc. there is now a nice "anchoring" shadow against the floor.

This effect normally only appears in a much longer render; AO allows you to simulate that effect with less render time.