Studied psychological correlates of irregularity in chronic sleep routines. The California Psychological Inventory (CPI) and Cornell Medical Index (CMI) were administered to two groups of 18 male university students who were categorized as either irregular sleepers or control Ss. The control group was composed of persons who habitually slept from 12-8:00 A.M. Irregular sleepers were defined as those whose retiring and awakening times continuously varied by about 2 to 4 hours. Control Ss scored significantly higher than the irregular group on the CPI scales of Do (dominance), Sy (sociability), Sa (self-acceptance), Sc (self-control), Ac (achievement via conformance), and Ie (intellectual efficiency), but lower on the Fx (flexibility) scale. There were no significant differences between the groups in scores on the CMI or average sleep length recorded over 2 weeks. The present findings indicate that in young adults, personality functioning is related more closely to the regularity of nocturnal sleep routines than to differences in chronic sleep duration. It is postulated that stable or irregular sleeping patterns are largely dependent factors not only of the psychological characteristics that distinguished the groups, but as yet unspecified constitutional and sociocultural antecedents of the human sleep response.