The NASA ISS Science Officer Ed Lu is writing kind of a diary when on his mission on the ISS. In the nowhere of the space, orbiting the earth.
"There really isn't an up or down anywhere else here, but there is a direction we think of as the floor and a direction we think of as the ceiling in each module. Most of the labeling on panels and equipment is written so that it is right side up assuming this orientation, and also most of the lights are on the "ceiling" so they cast light "downwards." To add to the effect, there is a simulator back on Earth we spent a lot of time in where we got used to one direction as the floor and the opposite direction as the ceiling. So up here, when Yuri and I say downwards or upwards, we mean the equivalent directions as in the training module on Earth."
But hopefully he won't have to buy the spacestamps.