Mulder, J. B. 1976. Behavior patterns of laboratory animals. Lab Animal 5(5), 22-28.

Placing of the cage on a rack even further reduces the available light. During experimentation, care must be exercised to use cages of the ... same placement to avoid variations in lighting. If this is not done, the investigator may be measuring behavioral rather than experimental results. Various resting levels are recommended for group-housed animals to account for the hierarchical order and the privilege of high ranking group members to occupy relatively high resting sites.