# Time Course of Sleep Inertia Dissipation in Memory Tasks
## 1 Introduction  
Sleep drunkenness [1], post-dormital sleepiness [2], or sleep inertia [3], as it is currently named, refers to a complex psychophysiological phenomenon, observed after awakening, that can be described as the gradual recovery of waking-like status [4]. This transition is not based on an all-or-nothing process, but acts according to slow and progressive mechanisms [5]. The first study on sleep inertia (SI) dates back to 1961 [6], and over the past 60 years, a considerable number of experiments has been carried out on this topic [4, 7]. SI magnitude, severity, and duration depend on several factors, such as prior sleep duration and deprivation [8, 9], sleep stage of awakening [10, 11, 12], and the time of day at which awakening takes place [13], as well as the type of task participants are accomplishing [14, 15]. The duration and time course of SI is one of the most controversial aspects in the literature on SI [7, 16], in particular regarding everyday activities in the normal sleep-wake condition. Cognitive processes are diversely sensitive to SI [7, 17], but it is still not clear whether all, and which, cognitive processes are mostly involved. Complex tasks seem to be less sensitive to SI compared to simpler ones [18]; in particular, strategic mechanisms seem to contrast a physiological condition of low arousal better than automatic ones, eventhough this datum has not been confirmed in subsequent studies [15; 19; 20]. Moreover, some higher-order cognitive processes are differentially sensitive to different sleep-wake regulatory processes [21]. The difference among the studies concerning the time course of SI can derive from methodological standpoints. First, only a few studies have investigated the time course of SI dissipation by measuring performances at regular time intervals and for a long time [22]. Indeed, most authors have typically made only one performance assessment after awakening (e.g., [23]). Therefore, SI has been reported to be short-lasting, rarely exceeding 30 min [18; 24]. Second, the SI effect in some studies was measured by comparing post-sleep performances with pre-sleep ones, while in other studies it was evaluated by recording the time needed for the performance to level off during the post-sleep period [7]. Third, SI varies according to the performance indexes (i.e., accuracy and/or speed) utilized. In some conditions, SI affects speed but not accuracy [25; 26], whereas both indexes decreased when sleep loss was associated with SI [24]. Finally, experimental data show differing results regarding the effect of sleep stage at awakening on SI. There is general agreement regarding a strong SI effect upon awakening from Slow Wave Sleep (SWS) [7; 27]. In some studies, SI did not significantly differ as a function of waking up during stage-2 (St2) vs. Rapid Eye Movement (REM) sleep [28]. Other studies reported a more deteriorated performance after awakening from St2 as compared to REM sleep [28; 29]. Thus, the SI effect on cognitive performance according to the sleep stage at awakening is still a matter of debate and further studies are needed. It is relevant to note that SI has been studied in applied fields (i.e., emergency conditions, shift work schedules, sleep disorders). Thus, participants have been tested in particular sleep conditions, such as sleep deprivation, sleep fragmentation, short naps, or SI during the first or second part of the night. By way of contrast, very few studies have investigated SI in a normal sleep-wake cycle condition [15; 28; 29; 30]. Importantly, the studies report different findings, restricting the generalizability of results. In light of the above-mentioned issues, the main purpose of this research is to evaluate an SI effect on memory performance after an uninterrupted night's sleep. As far as cognitive functions are concerned, in this study, we focused on memory performances. Mnemonic functions have been investigated sporadically.