Introduction: Napping is nearly universally recommended for night shift workers, with the rationale that additional sleep will reduce the associated sleepiness and performance deficits and improve safety. Indeed, night workers take naps before approximately 40% of their 8-hour night shifts. The present study systematically examines the effects of napping before five consecutive simulated night shifts (5NAP), or before the first two of five consecutive simulated night shifts (2NAP), as compared to a no-nap group (0NAP). The impact of these conditions upon the main daytime sleep period is the focus of this report; the effects on alertness and various performance measures during night shift hours are presented in a companion abstract. These reports are preliminary as additional subjects are being run and not all variables have been analyzed to date. Methods: Subjects were screened clinically and by PSG for sleep disorders and were required to have a mean sleep latency > 5 minutes on a screening MSLT. All were free of medical and psychiatric illness, and psychotropic medications. Shift workers or individuals with usual rise times after 0800 were excluded. Thirty-three subjects (14 m, 19 f; mean age 47 +/- 12.3) randomly assigned to one of the three nap conditions (N=11 for each) are included in this report. Sex representation was similar and mean age did not differ among groups. Each subject participated during five consecutive nights and the intervening four days. Evening naps were taken from 1930 to 2200. The simulated night shift, during which sleep was prohibited, began at 2300 and ended at 0735. All subjects left the laboratory from 0800 to 0830 during which time they were exposed to indoor sunlight. Daytime PSGs began at 0830 and ended with the subject's time-na´ve request after 1430, or at 1630. Results: Mean minutes of sleep during each of the nap opportunities did not differ within or between the nap groups: 92 and 103 minutes for the 2NAP group and 84, 79, 77, 67, and 63 minutes for the 5NAP group. Daytime sleep data are presented in the table. ANOVA indicated that daytime total sleep time (TST) differed significantly among groups (mean TST across 4 days was 374.5 min for 0NAP, 336.8 min for 2NAP, 292.8 min for 5NAP; p<.01). Post-hoc analyses showed that TST was significantly greater for 0NAP than for 5NAP for all sleep periods (p<.05); there was a trend for a difference in the same direction between 0NAP and 2NAP on day sleep periods 1 and 2 (p=.06). When 24-hour TST was calculated by adding minutes of sleep during the evening nap to TST for the subsequent daytime sleep period, no differences were found among groups; however, there was a linear decrease in TST across days (p<.01). Sleep efficiency (SE) was lower for 5NAP (73.0%) as compared to 0NAP (85.8%; p<.01). SE for 2NAP was lower than that for 0NAP for day sleep period 1 (p<.05) and there was a trend for SE to be lower on days 2 and 4. The longer TST for 0NAP, compared to 5NAP, was the result of significantly more stage 2 and REM. Conclusions: Evening naps of approximately 60-90 minutes duration appear to significantly interfere with sleep on the subsequent day. Uncontrolled studies have suggested that shift workers who nap have reduced daytime sleep quality, as compared to non-nappers. This may reflect a lower homeostatic drive, or may be related to social/behavioral influences. The significantly lower SE for daytime sleep following nights with naps, combined with the equivalent 24-hour TST for the three groups, suggest that there is a reduced homeostatic drive for sleep during the day following evening naps.