Past research has indicated that caffeinated 'functional energy drinks' (FEDs) are effective in counteracting sleepiness. It is not known however, what impact FEDs have on sleep itself. FEDs contain several active ingredients, including caffeine. They may therefore impact negatively on sleep and hence subsequent performance, deeming their use counterproductive. In a randomised cross-over design, 15 young adults participated in a simulated first night-shift protocol with 2 conditions, Functional Energy Drink (FED) and Non Functional Energy Drink (NonFED). Both involved a period of extended wakefulness (0700-0730 h-24.5 h) followed by an 8-h daytime 'recovery' sleep (0730-1530 h). During the FED condition, a commercially available FED was administered twice during the night. Sleepiness was assessed during the period of extended wakefulness and for a further 6h after waking. Sleep periods were recorded using a standard 5 channel polysomnogram. Comparison of the sleep periods showed that sleep onset latency remained unchanged as did stage 2 and slow wave sleep. Total sleep time however, was 29.1 min shorter (p<.05) in the FED condition. Sleep efficiency was also significantly reduced from 91.8+/-.9% to 84.7+/-2.7% (p<.05). It is evident that the residual effects of the FED's active ingredients impact on some aspects of daytime sleep following a simulated night-shift. Subsequent performance however was unaffected. The results deem FEDs to be effective for a single night-shift and warrant investigation into their use over successive night-shifts.