Nightshift is a common work schedule in health environments, and is associated with decreased alertness and increased adverse events at work. This decrease in alertness can be predicted from biological models of sleep homeostasis and circadian influences. Naps can provide a short-term alleviation of sleep need, and the benefits of naps have been demonstrated in laboratory-based studies, and in specific controlled work environments. The efficacy of brief naps has not been demonstrated in health workers in their usual work environment. The current study examined the effects of a 30-min nap break during the nightshift in a cohort of nursing staff and scientists in their usual work environment. Measures of both subjective sleepiness and objective alertness were taken at hourly intervals throughout the nightshift, on nights where a scheduled nap was taken and nights where there was no nap. Following a nap, psychomotor performance metrics (response speed and fastest 10% reactions times) improved and self-reported sleepiness was reduced compared to nights without a nap. These improvements persisted to the end of the nightshift. Effect size estimates suggested that 20–50% of variance in these measures was explained by the interaction of time on shift and the nap intervention. Differences were found in the timing of increases in subjective and objective alertness after the nap. These findings support the effectiveness of a scheduled nap break during a nightshift to maintain alertness in health workers.