After a night of normal sleep, 36 subjects were divided into three groups of 12. One group was allowed to continue sleeping normally, one group had four hours continuous sleep and the third group had three 80-min periods of sleep distributed throughout the 24-h period, for the next four days. Subjects were given a visual vigilance test on each day. Overall performance was rather better following distributed than following continuous reduced sleep, but rather worse than following 8 h of sleep. The main difference between the reduced sleep groups was in their decision criterion. It was suggested that this difference mainly reflected the difference in time since the two groups had last slept.Now at the Department of Psychology, University of Otago, New Zealand.