Abstract
A set of 23 inbred strains of mice was tested for their sleeping time under sodium pentobarbitone anaesthetic. Highly significant strain differences were found. Estimates of the proportion of the variation accounted for by genetic differences ranged from 28% to 42%. In general, males slept longer than females but the size of the sex differences was not consistent across strains. Sleeping times on different test days also varied, indicating that environmental factors were affecting the results. A specially designed experiment failed to detect any differences in within-strain variation.