Sleep plays a crucial role in mental and physical health throughout one’s lifespan. On the one hand, sleep loss worsens mood and decreases the ability to regulate negative emotions ^[1][2][1,2]. It increases negative emotions and emotional reactivity, reduces the experience of positive emotions, and alters how individuals understand, express, and modify these emotions ^[3][4][3,4]. Sleep deprivation may even impede the effectiveness of adaptive emotion regulation, such as distraction and cognitive reappraisal, consequently impacting emotional well-being ^[5][6][5,6]. On the other hand, daily emotional stress is also known to affect subsequent sleep ^{[7][8][9][10][11]}[7,8,9,10,11]. Emotional stress induced by an emotional failure experience does not only result in high negative affect, but can also affect sleep efficiency (SE), sleep onset latency (SOL), wake after sleep onset (WASO), total time awake, number of awakenings during the night, number of awakenings from rapid eye movement (REM) sleep (dream sleep characterized by REM, but also low muscle tone and rapid, low voltage waves), % REM-sleep, and % slow wave sleep (SWS) [10]. Although it has been consistently shown that emotional stress can elicit profound and lasting effects on sleep (for a review, see [12]), the moderating effects of emotion regulation remain relatively less understood.

Emotion regulation refers to the process by which an individual influences the nature of his or her emotions and how emotions are experienced and expressed ^[13][14][13,14]. Individuals may not always hold the ability to control the stress factors they encounter in life, but they may be capable of learning to adopt efficient emotion regulation strategies, and thus control the detrimental effects of stress on their sleep activity. Although often based on subjective reports, there is a growing interest in the potential role of emotion regulation in response to daily emotional events on sleep quality and vice versa (for examples, see ^[5][15][5,15]). However, prior research mostly focused on traditional top-down emotion regulation strategies, such as cognitive reappraisal and distraction, and less on bottom-up emotion regulation strategies, such as experiential emotion regulation. As a complementary more bottom up emotion regulation strategy, experiential emotion regulation involves an active, non-intervening, accepting, open and welcoming compassionate approach towards the bodily felt sensory affective experience or ‘experiential awareness’ in a first phase, and its verbalization or ‘experiential expression’ in a second phase. To address this gap in the literature, reseaourchers' lab conducted a study to investigate whether experiential versus cognitive emotion regulation strategies moderate the relationship between emotions elicited by a painful failure experience and subsequent sleep differently [11]. Participants who were instructed to deploy experiential emotion regulation were asked to focus on their low-level and concrete affective experience by affectively acknowledging and understanding it in the context of the specific situation, and expressing it in an open, non-evaluative mode. Compared to participants deploying a cognitive emotion regulation strategy, those in the experiential emotion regulation condition took longer to fall asleep, but experienced significantly fewer awakenings, showed longer total sleep time (TST), and higher sleep efficiency (SE). The present researchtudy aimed to further validate the impact of experiential emotion regulation versus cognitive reappraisal on pre-sleep emotional experience and follow-up sleep physiology, both relative to a control condition.

A well-studied cognitive emotion regulation strategy is cognitive reappraisal. Cognitive reappraisal is defined as the attempt to reinterpret an emotion-eliciting situation in a way that alters its meaning and changes its emotional impact [16]. One approach to top-down cognitive reappraisal, ‘cognitive reinterpretation’, is a cognitive-linguistic strategy that alters the trajectory of emotional responses and diminishes its impact by changing the meaning of an emotion through reframing the context of the emotion-evoking stimulus [13] (e.g., “She did not say hello because she was distracted, not because she does not like me”). The theoretical models underlying cognitive emotion regulation, i.e., the process model of emotion regulation ([17]; for an alternative, see [18]), consider cognitive reappraisal as a linear top-down process. Consequently, most of the insights gained in the cerebral architecture of cognitive emotion regulation concern the downregulation by, and interplay between, prefrontal control circuits and limbic emotion generation circuits, such as the amygdala, striatum, and insula ^[19][20][21][19,20,21]. Given the adaptive effect of cognitive reappraisal on the modulation of emotional stress, rwesearchers assume that cognitive reappraisal would also have a beneficial effect on sleep in the present researchstudy. However, research on the impact of cognitive reappraisal on sleep shows inconsistent findings. Some researchers found that poorer sleep quality is associated with a lower ability to cognitively reappraise [22]. Conversely, individuals who are more capable of adopting cognitive reappraisal in their daily lives are more likely to have enhanced sleep quality [23]. On the other hand, the impaired quality of sleep influences the use of emotion regulation, such as expressive suppression. According to Ellis, Prather, Grenen, and Ferrer (2020) [24], sleep quality was indirectly related to the habitual use of cognitive reappraisal. Moreover, other research demonstrated that sleep quality is not related to cognitive reappraisal ability ^[25][26][25,26]. Altogether, it is still unclear how cognitive reappraisal impacts pre-sleep emotional stress and thus contributes to follow-up sleep.

Experiential emotion regulation involves an active, non-intervening, accepting, open and welcoming approach of acknowledging and gaining awareness of raw sensory affective experiences or ‘experiential awareness’ in a first phase and expression in a second phase. Instead of emphasizing an active and cognitively controlling way of coping with emotions, experiential emotion regulation highlights the importance of the affective process itself, as an adaptive signaling mechanism and a bottom-up pathway to process emotional experiences in more depth. Originating from humanistic, client-centered and experiential psychotherapeutic approaches, such as Experiential Focusing ^[27][28][29][27,28,29], experiential emotion regulation primarily focuses on the importance of experiencing one’s feelings and emotions in the immediate present to achieve emotional change. ‘Experiencing’ involves the awareness of an emotionally tinged experience together with its personal meaning [27]. Furthermore, experiential emotion regulation involves an accepting, non-intervening, open, and welcoming approach towards the present affective sensory feeling ^[28][29][30][28,29,30]. Research demonstrated that participants who were instructed to apply an ‘experiential self-focus’ on the concrete “what” or content of the feeling or affective experience were more adaptive to recover from painful life events compared to participants with a cognitive analytical “why” focus [31]. As illustrated by these findings, an experiential approach enhances people’s ability to face bottom-up generated stressors involving a bodily felt affective experience, leading to integration and acceptance of predictable stressors and reduced arousal levels from the stressors in the long-term.

In daily life, emotional stressful life events mostly involves repeated or persistent experience or confrontations with the same or similar emotional events (e.g., the memory of a deceased loved one, a divorce, or failing an important exam). Yet, in current emotion regulation research, only a few studies investigated the effects of repeated or sustained emotion regulation, which was also limited to cognitive strategies. For instance, Erk et al. (2010) [32] investigated the neural activity associated with the temporal dynamics of acute and sustained cognitive emotion regulation in patients with major depressive disorder and healthy controls using functional MRI (fMRI). Only amongst healthy controls, sustained cognitive emotion regulation was associated with a reduced activation in the amygdala after a 15 min delay. In a study by Denny et al. (2015) [33], cognitive reappraisal resulted in decreased negative emotion and amygdala activation, which remained attenuated for emotional images that had been reappraised four times, compared to images that were reappraised once, new control images, and control images that were never reappraised. Another study showed that repeated cognitive reappraisal resulted in reduced negative feelings and stronger dorsolateral and ventrolateral prefrontal cortex responses one day later [34]. Importantly, acute versus repeated emotion regulation effects may be different for cognitive and experiential emotion regulation. When applying a cognitive approach to emotional stressors, the emotion itself might be immediately regulated [11]. Conversely, experiential emotion regulation may initially enhance affective intensity and reactivity when processing a negative emotional stressor, whereby only repeated processing of the same or similar emotional stressors may result in profound regulation and recovery ^[12][35][12,35]. This adaptive regulation and recovery processes are expected to result in less emotional intensity and less negative evaluations of similar stressful events in the future.