In the present study, we analyzed ERPs to investigate the effects of 8 h of RS on working memory impairments induced by 36 h of TSD. Our previous results revealed that TSD significantly impairs accuracy in 2-back working memory tasks . The current behavioral findings demonstrate that 8 h of RS can improve both accuracy and the number of correct responses per unit time in such tasks. Changes in behavioral indicators intuitively reflect increases in working memory ability. Although a constant cognitive load (2-back) was utilized in the present study, our results indicated that working memory capacity had improved following 8 h of RS, when compared with the capacity observed following 36 h of TSD without RS. However, emphasizing accuracy rates can also influence response times . Although no improvements in reaction time were observed following 8 h of RS, the number of correct responses per unit time significantly increased. Therefore, we believe that the number of correct responses per unit of time more accurately reflects the individual's working memory ability and level of cognitive control.
Notably, 8 h of RS following TSD also induced significant decreases in N2/P3 latency, as well as significant increases in P3 amplitude. The N2 component is considered to reflect an individual’s mental state and level of attention , while the P3 component is thought to be involved in the decision-making process during cognitive-matching tasks . Increases in P3 latency and decreases in P3 amplitude have been associated with prolonged wakefulness . Numerous studies have reported that reaction time and sustained attention decrease following SD [50, 51]. Thus, in accordance with previous results, our findings support the notion that 8 h of RS can improve performance and alertness . Restoration of attention and alertness following RS may have enabled participants to allocate more attentional resources to the working memory tasks, thereby attenuating TSD-induced impairments . Previous studies have also indicated that, relative to drowsiness, SD is associated with more pronounced decreases in activation of the frontoparietal network involved in working memory . Furthermore, SD can reduce metabolic activity in regions associated with information processing and executive control , while RS can restore overall network organization following TSD .
Sustained attention and alertness are essential for performing daily activities. Based on the observed changes in the N2 and P3 components (i.e., increased amplitude and decreased latency) after 8 h of RS, we speculate that RS can effectively attenuate impairments in attention and alertness, thus influencing the information integration process. The deterioration of sustained attention seems to be the most long-lasting negative effect of SD [55, 56,] likely due to a decrease in general central nervous system (CNS) arousal [57, 58]. In contrast, more automatic or bottom-up processes appear to be less affected by changes in CNS arousal . Therefore, improvements in sustained attention are likely to occur earlier given the greater sensitivity of sustained attention to SD. The completion of a cognitive task usually requires the joint participation of several psychological processes, including early sensory perception, alertness, basic attentional mechanisms, working memory, and decision-making. The P3 component appears relatively late, suggesting that it is more reflective of conscious participation, which likely involves top-down cognitive control . The observed increases in amplitude and decreases in latency also suggest that 8 h of RS improves the ability to integrate dynamic information during working memory tasks. Communication between the hippocampus and prefrontal areas is vital for optimal redistribution of temporal memory traces to more resident cortical storage, and interrupting this communication may impair the ability to form a new memory. However, Chai et al. recently noted that RS re-normalizes these hippocampal connections .
Normal sleep is divided into two phases: rapid eye movement (REM) and slow-wave sleep (SWS). Deep sleep during the N3 stage of SWS is particularly important for restoring mental and physical energy. Following SD, compensatory responses are observed during the restorative stages of sleep. Interestingly, it is the intensity rather than the duration of sleep that influences the recovery of function following SD. Sleep intensity during SWS is regarded as an indicator of homeostatic sleep stress [29, 60]. After one night of SD, less than 10 h of RS can sufficiently decrease the level of sleep stress to that observed at the end of a typical 8-h period of normal sleep [61, 62]. Nevertheless, further increases in the duration of SWS are observed on the second night of recovery . RS exhibits characteristics distinct from those of normal sleep, including decreases in sleep-onset latency. Key changes are also observed during the N2 and N3 stages. Longer periods of RS result in a more similar proportion of time spent in sleep stages between normal sleep and RS . Therefore, we speculate that individuals experienced increases in the proportion of SWS during RS, relative to the amount observed during normal sleep. It may be that SD-induced impairments in working memory function are specifically attenuated during SWS.
Although the present behavioral and EEG data support our hypothesis that 8 h of RS can attenuate impairments in working memory caused by 36 h of TSD, we did not observe significant changes in all indicators identified in our previous study . Insufficient sleep may lead individuals to provide conservative estimates of their performance, which may increase the likelihood of compensatory behaviors and protect against the negative consequences of SD . Therefore, the results of our study should be interpreted with caution. Studies have further demonstrated that simple cognitive responses are less affected by SD and can be easily recovered following RS, while impairments in higher-level cognitive functions are less easily reversed [64, 65]. Improvements in cognitive function following RS are mainly reflected by changes in alertness and sustained attention, allowing participants to allocate more attentional resources to the current task . Nonetheless, further studies are required to elucidate the mechanisms by which RS restores cognitive function following TSD.
The present study possesses some limitations of note. First, we did not assess working memory performance using tasks of varying difficulty, limiting our ability to infer how changes in workload impact the restorative effect of RS. In addition, our study included male volunteers only, necessitating caution when attempting to extend the findings to female individuals. Considering the small number of participants in our sample and some non-significant findings related to EEG indicators, further studies are required to determine whether 8 h of RS can restore cognitive function to baseline levels. In our future studies, we plan to include a control group to improve the rigor of our experimental design. Multimodal studies involving brain network analyses of EEG and imaging data may help to further explain our results. We did not utilize sleep monitoring technology to investigate whether RS induces specific alterations in sleep structure, necessitating further studies. Finally, given that circadian biorhythms affect behavioral performance , their effects cannot be excluded.
Our results align with those of previous studies, suggesting that 8 h of RS can partially attenuate the deleterious effects of TSD on working memory. Failure to maintain a level of alertness during military missions may lead to serious consequences. Under the conditions of future high-tech warfare, the problem of sleep deprivation may become more prominent. The widespread use of high-tech equipment puts forward higher requirements on the cognitive ability of the brain, therefore, it is of great military significance to attach importance to and strengthen the research on sleep deprivation and medical support under continuous combat conditions.