The vigilance states and motor activity of the rat were recorded during ultrashort light-dark (LD) cycles. The LD schedules were the following: 10 min light -- 10 min dark (LD 10:10), LD 7.5:7.5, LD 5:5 and LD 2.5:2.5. With all short LD schedules, paradoxical sleep (PS) and high motor activity occurred more frequently during dark periods than during light. PS and motor activity were rapidly enhanced by darkness and inhibited by light, whereas the percentages of total sleep and slow wave sleep were only gradually decreased during the dark periods and increased during light. Compared to the LD 12h:12h control days, sleep was enhanced by all short LD schedules, an effect that was most marked in the circadian phase of maximum waking (corresponding to the LD 12h:12h dark period). The short LD cycles interacted with an intrinsic sleep cycle, whose period is approximately 10 min. Interval histograms of PS episodes showed that an optimal synchronization of the ultradian sleep rhythm is achieved when the period of the LD cycle or its multiple correspond also to 10 min. It is suggested that the response of PS and motor activity to changes in illumination may be due to common mechanisms.