The data from all three studies indicated that individuals who take a prophylactic nap and use caffeine during their work shift will have significantly increased objective alertness during the nocturnal work shift. It would be predicted that the potential for a catastrophic mistake would be the greatest when an employee was awakened and immediately forced to make an important decision. Although such a situation was not explicitly measured in Study 3, it is common that individuals allowed to sleep during work situations will be awakened only when an emergency occurs. Several studies have indicated that performance is worse immediately after awakenings during nocturnal sleep periods ( Dinges, 1989; Stampi, 1989). Sleep inertia effects are increased after awakenings from slow wave sleep ( Bonnet, 1983) and have been found in many tasks including simple and choice reaction time ( Feltin & Broughton, 1968; Scott & Snyder, 1968), short term memory ( Bonnet, 1983; Stones, 1977), and letter substitution ( Dinges, Orne, & Orne, 1985). In the current study, 60% of nocturnal naps (vs. 10% of prophylactic naps) ended in slow wave sleep. This figure is probably a low estimate for the real world where on-call situations and other stresses typically cause chronic as opposed to acute sleep deprivation. The high percentage of awakenings from deep sleep implies that individuals in nocturnal on-call situations will frequently suffer from the maximum negative consequences of sleep inertia, sleep deprivation, and declining circadian rhythm.
Study 3 indicated that individuals who nap during the night in their work situation and who are awakened frequently or accumulate less than 4 hours of sleep will be less alert than individuals who take an afternoon nap and remain awake during the night. In terms of practical use, the results suggest that taking a nap prepares individuals to deal with the sleep loss that occurs during nocturnal work periods and that caffeine probably reverses some of the normal circadian decline in nocturnal alertness. However, this strategy also has limitations, and extending work periods into the afternoon or evening would have required additional sleep or caffeine administration.
In summary, these studies have demonstrated the beneficial effect of obtaining additional sleep prior to periods of sleep loss and the additive benefits of caffeine use. Subjects for the studies were healthy young adults who did not report significant daily caffeine use and who were not sleep deprived prior to the beginning of each study. In the real world, of course, it is common that shift workers will suffer from both chronic sleep loss and tolerance to caffeine. Such limitations could reduce the beneficial impact of both prophylactic naps and caffeine in applied settings. On the other hand, knowledge of the ability of naps and caffeine to produce baseline levels of alertness across nocturnal work periods when used correctly should allow planners to develop improved sustained work schedules.