The purpose of this study was to investigate the effects of a 30-min nap, during a simulated nightshift environment, when a prophylactic daytime sleep was implemented prior to the night shift. A repeated-measures counter balanced design was used which included two experimental conditions: a 30-min nap and a no nap control. In both conditions subjects obtained a 2-h sleep in the afternoon from 15.00–17.00 hours, which was followed by the night-time nap from 02.30–03.00 hours in a controlled laboratory environment. Post-nap testing was conducted from 03.10 to 07.00 hours. The participants included 22 adults aged from 18–35 years who were good sleepers and did not regularly nap. Subjective alertness (Stanford Sleepiness Scale, Karolinska Sleepiness Scale, Visual AnalogScale), fatigue and vigor (Profile of Mood States), cognitive performance (psychomotor vigilance task, symbol–digit substitution task, letter cancellation task), and objective sleepiness were measured preandpost-nap. The 30-min nap resulted in some impairment of subjective alertness for a brief period (up to 30 min) immediately following the nap when compared to the no nap condition. Following this brief period, alertness and performance were generally improved by the 30-min nap from 04.00 hours until the end of the testing period at 07.00 hours. The results of this study indicate thatwhen a 2-h prophylactic sleep is implemented in the afternoon, a 30-min nap during the subsequentsimulated night shift overall provides a significant countermeasure against sleepiness and performance impairment.