Ten regular 12-8 a.m. 8-hour sleepers performed a 5-min experimenter-paced calculation task, a 30-min vigilance task and completed an adjective check list to rate their mood following a 12-8 a.m. habitual sleep condition and 8 p.m.-4 a.m., 10 p.m.-6 a.m., 2-10 a.m., and 4 a.m.-12 p.m. conditions of shifted sleep. After the shifted sleep conditions compared to the 12-8 a.m. condition performance on the vigilance and calculation tasks was significantly impaired, and negative affect was significantly greater as measured by three scales of the adjective check list. Sleep length did not differ between the various conditions and the decrements in performance and mood were unrelated to any specific changes in the electrophysiological patterns of sleep.