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Aims: The aim of this study was to examine the effectiveness of a scheduled 30-min nap and its interaction with individual factors on sleepiness and cognition during an 8-hr night shift., Design: This prospective, within-subjects study conducted between 2011-2014 compared sleepiness and cognition with/without a nap during the night shift, in 109 female nurses, tested on two nights with and two nights without a nap in counterbalanced order., Methods: Nurses completed the Munich ChronoType Questionnaire for Shiftwork, Pittsburgh Sleep Quality Index and Pre-Sleep Arousal Scale at study onset. They reported sleepiness hourly and performed the Digit Symbol Substitution and the Letter Cancellation Tasks at 3:00 and 7:00 a.m. They took a nap at 4:00 a.m. on nap nights and worked as usual on no-nap nights. Sleep-wake patterns were monitored using actigraphs 24 hr before and during the shift. Caffeine consumption, workload and adverse events were reported. To assess the effectiveness of a scheduled nap, mixed-models and repeated measures analyses of variance were used., Results: Lower levels of sleepiness were found at 5:00, 6:00 and 7:00 a.m. on nap versus no-nap nights. Increments in performance between 3:00-7:00 a.m. were significantly greater on nap versus no-nap nights for Digit Symbol Substitution Task correct responses and Letter Cancellation Task capacity. No interactions between the nap and any of the individual factors emerged., Conclusion: A scheduled nap provides an effective countermeasure against the negative consequences of night-time shift work in female nurses above and beyond interpersonal differences., Impact: Changes in attitude and policy are required to implement this beneficial and cost-effective strategy., Trial Registration Number: ACTRN12618001857291., (© 2019 John Wiley & Sons Ltd.)

Decline in cognitive functioning in the workplace is a major concern for health care systems. Understanding factors associated with nighttime functioning is imperative for instituting organizational risk management policies and developing personalized countermeasures. The present study aims to identify individual factors associated with cognitive functioning during the night shift of hospital nurses working on irregular rotating-shift schedules. Ninety-two female nurses were recruited from 17 wards in two general hospitals, using convenience sampling by clusters. Inclusion criteria were working at least 28 h a week (75% of full time) and one night shift per week. Exclusion criteria were pregnancy, diagnosed sleep disorders or medical conditions that may affect sleep and/or function. Cognitive performance was measured during the middle (03:00 h) and at the end (07:00 h) of the night shift using the Digit Symbol Substitution Task (DSST) and the Letter Cancellation Task (LCT) over two night shifts. Subjective sleepiness was assessed by the Karolinska Sleepiness Scale (KSS) at the same time points. All participants completed a sociodemographic questionnaire, the Munich ChronoType Questionnaire for Shift-Workers (MCTQShift) and the Pittsburgh Sleep Quality Index (PSQI). Sleep duration 24 h before the night shift and time awake since last sleep opportunity were monitored by actigraphy. Univariate repeated measures ANOVA found main effects for clock time (p<0.001), age (p<0.05), time awake (p<0.05) and sleepiness (p<0.01) for DSST correct responses; main effects for clock time (p<0.001) and sleepiness (p<0.001) for LCT capacity; and main effects for clock time (p<0.001) and age (p<0.01) for LCT omission errors. All factors remained significant in a mixed-model analysis for DSST. Cognitive performance among hospital nurses is low during the middle of the night shift and increases at the end of the shift; decreased functioning is associated with increased subjective sleepiness, older age and prolonged time awake. Identifying factors contributing to performance during the night shift may provide a basis for the development of risk management policies and preventative interventions.