Nocturnal sleep plays a vital role in human memory consolidation (Newbury and Monaghan, 2019, Ackermann and Rasch, 2014). The relationships between sleep and sleep-dependent memory (Stickgold, 2005, Diekelmann et al., 2009), as well as emotional memory (Rihm& Rasch, 2015; Schafer et al., 2020), have been extensively studied. Sleep loss, including sleep deprivation or sleep restriction, can interfere with memory processing while getting enough sleep can promote memory performance (Newbury et al., 2021). Although previous studies have focused on the effects of acute sleep deprivation on memory, people who suffer from insomnia could also experience chronic sleep deprivation. Yet, the effects of insomnia-induced sleep deprivation on memory have been scarcely investigated.

Working memory (WM), an ability to store and manipulate information in a temporary memory buffer, is the basic requirement for higher executive function (Baddeley, 1992). The prefrontal cortex and amygdala, which are the two main neural structures involved in WM and emotion processing, are impaired by sleep deprivation (Goldsteinet al., 2015; Chee&Choo, 2004). An emotional stimulus can capture more attention and automatically activate neural responses (e.g., Amygdala modulation of hippocampal function) (Ramponiet al., 2011). Yet, little is known about the effects of emotion on WM, and the relationship between sleep deprivation and emotional WM has been scarcely investigated.

Several studies have investigated the effects of acute sleep deprivation on WM (Frenda and Fenn, 2016, Drummond et al., 2012). One early study has reported that total sleep deprivation impaired emotional WM regardless of the emotional valence of the stimulus (Tempesta et al., 2014). In contrast, Gerhardsson et al. (2019) reported that sleep-deprived participants responded faster to positive stimuli than to negative and neutral ones. These findings provided us with important insight into the link between emotional WM and sleep. Yet, participants employed in these studies experienced experimentally induced sleep deprivation. Whether and to which extent insomnia-induced chronic sleep deprivation or disturbance affects emotional WM has been scarcely investigated.

Insomnia disorder (ID) is one of the most common sleep disorders, which is mainly characterized by difficulty falling asleep, difficulty maintaining sleep, early awakening, poor sleep quality, and impaired cognitive function during the day (American Academy of Sleep Medicine, AASM, 2017). Recently, one study by Xi et al. (2019) exploring the effect of insomnia on long-term emotional memory has reported an emotional valance dependency in insomnia-induced impairment of long-term memory, showing that worse memory performance was rendered for positive and neutral but not for negative stimuli. Thus, the first aim of the current study was to explore the effects of insomnia on WM of emotional stimuli across different values.

Memory consists of encoding, retention (consolidation), and retrieval. Ericsson et al., (1995) first introduced the concept of “long-term working memory,” a set of “retrieval structures” that allows people to seamlessly access everyday information from long-term memory. This implies that certain components of long-term memory effectively serve as WM. Emotional information could lead to memory enhancement at various phases of memory (LaBar& Cabeza, 2006). As the long-term situational memory approach has been employed, the current literature on the effect of sleep deprivation on WM particularly focused on memory encoding and retention (Tempesta et al., 2018). To date, it still remains unclear whether insomnia-induced sleep deprivation or disturbance affects emotional working memory performance, and at which phase these effects manifest.

A delayed recognition task created by Morgan et al. (2008) could provide the possibility to divide the cognitive process of WM into encoding, retention, and retrieval phases. Moreover, the high temporal resolution of the event-related potentials (ERPs) has been widely recorded to explore the temporal dynamics of neural activities involved in different phases of cognitive processing of WM (Campanella et al., 2004). Therefore, the second aim of the current study was to investigate the neurobehavioral responses of ID involved in encoding, retention, and retrieval of emotional WM by adopting the delayed recognition task and ERP approach.

Referring to the studies by Morgan et al., (2008) and Zhang et al., (2019) assessing the neural correlations involved in different processing phases of WM, the interested ERP components, including P1 in the occipital area, N170 in the temporo-occipital area, P3 in the parietal area, and negative slow wave (NSW) in the prefrontal region, were investigated in the current study. P1 component is associated with the early visual processing of attention work (Batty＆Taylor et al., 2003; Kalsi et al., 2019), while it is also modulated by emotional information (Schupp et al., 2006). N170 is a reliable electrophysiological indicator of facial processing (Hinojosa et al., 2015). Additionally, facial expression information affects the structural coding of the face, and the expression-triggered N170 amplitude is larger than that of neutral faces (Rellecke et al., 2013). In a study by Cote's (2014), the sleep-deprived participants showed increased N170 amplitudes for Fearful and Angry faces as the difficulty of perceiving the emotions increased. P3b reflects cognitive processing of emotional cues, response-related, and decision-related stimuli (Schupp et al., 2006). The early P3b and late P3b as two subcomponents of P3b reflecting stimulus evaluation, and memory search operations, respectively (Morgan et al., 2008). Bistricky et al. (2021) reported that individuals experiencing dysfunctional sleep insufficiency exhibited reduced average P3 amplitudes in response to positive facial expressions. NSW reflects the retention and repetition of information in the brain (Ruchkin et al., 1995) and continues throughout the memory retention phase (Drew et al., 2006).

In addition to the above-mentioned emotional valance induced moderation of ERP component, fewer studies have documented that insomnia showed increased attentional bias towards negative stimuli. One early study by Baglioni et al. (2014) reported that individuals with insomnia exhibited heightened neural reactivity to negative emotional stimuli, particularly in the amygdala, which is a key brain region involved in emotion processing. Another study utilizing the emotional Stroop task reported that participants with insomnia showed attentional bias toward sleep-related negative than positive information (Zhou et al., 2018). Thus, it was hypothesized that the SID group vs. the HC group would show an emotional valance-dependent in WM performance of emotional faces. Thus, the SID group vs. the HC group would pay more attention to negative faces, showing smaller P3b and larger N170. Compared with neutral faces, negative faces would elicit larger P3b, N170, and NSW for participants in both SID and HC groups.