The present study examined the effects of regularizing sleep–wake schedules on sleep, autonomic function and mood/emotional and personality states in 14 habitually irregular sleepers. During the experiment, sleep monitoring and regularized sleep–wake schedules were conducted at home. First, the subjects' habitual sleep–wake patterns were strictly monitored for 6 days (Session 1); second, subjects' irregular sleep–wake patterns were regularized for 6 days (Session 2); and finally, subjects reverted from their regularized sleep–wake schedules to their original, habitual sleep–wake schedules and were monitored for 6 days (Session 3). Assessments in the laboratory, which were conducted in the daytime, were repeated four times. The first three assessments were carried out on the days following Sessions 1, 2, and 3. The fourth measurement (Session 4), which was conducted under the original irregular sleep–wake pattern condition, was performed approximately 6 months after initiation of the second irregular sleep–wake schedule. In results, significant reductions in daytime parasympathetic activity and negative mood, i.e. tension–anxiety, anger–hostility, and fatigue, were observed after the 6-day regularized sleep–wake schedule and extending through Session 3. However, the parasympathetic activity, as well as negative mood manifestations, returned to their initial levels when measured 6 months later. On the other hand, sleep and personality states did not change in response to alternations in sleep–wake regularity. However, the reduction of daytime fatigue level suggests a possibility that further sleep improvements might be difficult to detect under the regularized sleep–wake schedule because the subjects were healthy university students without sleep disorders.