Considerable efforts have been made over the last few decades to standardise the housing and care of laboratory animals. This has been done to try to reduce variability in responses to experimental manipulations and to produce more uniform base-line data. Laboratory animals are housed in standardised caging with standardised types of bedding and fed a standard, well-characterised diet. Room environmental conditions are maintained within prescribed ranges of temperature, relative humidity and number of air changes, and with a fixed light-dark cycle. These factors can also be influenced by the frequency of cage cleaning, and by the use of environmental enrichment, so these practices also require standardisation.