Introduction: Alertness and performance of most skills in humans decline to their lowest daily levels during usual night shift hours. Even after weeks or months on the night shift, physiologic adaptation to night work is incomplete at best, and often is minimal. Practical countermeasures to enhance alertness and performance on the night shift are needed, particularly for those occupations key to public safety. 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 alertness and various performance measures during night shift hours is the focus of this preliminary report. Methods: Subject selection criteria and the schedules for daytime sleep and naps can be found in a companion abstract in this volume. 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. The simulated night shift, began at 2300 and ended at 0735, during which time the Maintenance of Wakefulness Test (MWT; 2345, 0210, 0420 and 0645), a Neurobehavioral Assessment Battery (NAB; 2300, 0125, 0335, 0600), and various cognitive tests (0040 and 0515) were administered. Only the MWT and some variables from the NAB are reported below. Results: Mean minutes of sleep during each of the nap opportunities were: 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 elsewhere. Data analysis consisted of mixed-model ANOVA: 3 groups x 5 nights x 4 time points per night. MWT analysis indicated a main effect for time-of-night on the MWT (p<0.01), with latencies becoming much shorter as the night shift progressed. There was a night x group interaction (p<.01), a time-of-night x group interaction (p<0.01) and a trend for a night x time-of-night interaction (p=.054). Compared to 0NAP, the 2NAP group was significantly more alert on night 1 (mean latencies 14.8 min for 0NAP and 27.4 min for 2NAP, p<.01) and tended to be more alert on night 2 (0NAP = 18.2 min, 2NAP = 26.7 min. p=.055); there was a trend for 5NAP to be more alert on night 1 than 0NAP (0NAP=14.8 min, 5NAP=22.0 min, p=.07). 2NAP was more alert than 5NAP on night 2 (2NAP=26.7 min, 5NAP=17.3 min, p<.05). No group differences were seen for the MWT on nights 3, 4, or 5. Lapses on the psychomotor vigilance test (PVT, part of the NAB) increased during the night as well as across nights (p<.01 for both). However, there was no main effect of group, nor were there significant interactions. Data were similar for the slowest 10% of responses on the PVT. The Digit Symbol Substitution Test (DSST) showed significant main effects for group (p<.05), night (p<.01), and time-of-night (p<.01); 5NAP performed better than 2NAP on every night but 0NAP did not differ from either group. DSST performance decreased slightly across the night but improved from night 1 to 5. The fatigue scale of the Profile of Moods Scale (POMS) and a visual analog scale of sleepiness showed significant time-of-night effects (p<0.01 for both). Neither had a significant group effect or interaction. Conclusions: These preliminary analyses suggest a modest positive effect of evening naps on alertness and neurobehavioral performance, limited to the first of five consecutive simulated night shifts. The robust time-of-night effects on all measures reported indicate that the dependent variables employed are quite sensitive to fluctuations in alertness/performance. Examination of the total time slept within the 24-hours prior to each night shift showed no differences among groups, suggesting that evening naps reduced sleep time during the main daytime sleep period the following day. Based on these preliminary analyses, the failure of evening naps to represent additional sleep in this simulated night work design probably accounts for the minimal effect of naps on night shift alertness and performance.