In their target article, Mahr & Csibra (M&C) propose a compelling and intriguing account of episodic memory as an essential part of human social interaction and communication. They suggest that episodic memory is tailored to provide reasons for people's beliefs and interpretations of the past in the communication with others. We propose that sleep may play an important role in this social dimension of episodic memory.

Sleep is well known to support the consolidation of episodic memory (Diekelmann & Born Reference Diekelmann and Born2010). Episodic experiences are retained more successfully in memory if the experiences are followed by a period of sleep compared to an equivalent period of wakefulness (Rasch & Born Reference Rasch and Born2013). In line with the active system consolidation account, sleep initially strengthens and stabilizes episodic experiences in the autobiographical context with rich spatiotemporal information (Inostroza et al. Reference Inostroza, Binder and Born2013). Over subsequent sleep periods, possibly spanning several days or weeks, sleep then transforms these detailed episodic memories into more schema-like representations for the long term (Lewis & Durrant Reference Lewis and Durrant2011). Sleep thereby abstracts and generalizes single episodic memories, leading to the generation of new gist knowledge (Lutz et al. Reference Lutz, Diekelmann, Hinse-Stern, Born and Rauss2017) and, in some cases, even to the development of false memories (Diekelmann et al. Reference Diekelmann, Born and Wagner2010; Payne et al. Reference Payne, Schacter, Propper, Huang, Wamsley, Tucker, Walker and Stickgold2009). In this consolidation and transformation process, sleep particularly facilitates memories that are important to the individual and relevant for future behavior. Thus, sleep preferentially supports memories that are emotionally salient (Payne & Kensinger Reference Payne and Kensinger2010), that are expected to be tested after sleep (Wilhelm et al. Reference Wilhelm, Diekelmann, Molzow, Ayoub, Molle and Born2011), that are expected to be rewarded (Fischer & Born Reference Fischer and Born2009), and that are associated with a plan to be executed in the future (Diekelmann et al. Reference Diekelmann, Wilhelm, Wagner and Born2013).

These essential functions of sleep for episodic memory may build the foundation for successful social interaction and communication with others during the day. Based on the notion that humans constantly represent themselves in the context of their surrounding social world (Müller-Pinzler et al. Reference Müller-Pinzler, Krach, Krämer and Paulus2017), we propose that episodic memories about others (and possibly also about ourselves during those episodes with others) are processed and abstracted during sleep. The vast information about the encounters with others initially is stored in the form of single episodic memories (e.g., one person telling the truth in situation A, while another person behaves awkwardly in situation B). These single episodic memories are presumably also laden with an emotional tone that was experienced during the social interaction, ranging from basic emotions such as joy or fear to specific interpersonal emotions such as embarrassment or pride (Müller-Pinzler et al. Reference Müller-Pinzler, Gazzola, Keysers, Sommer, Jansen, Frässle, Einhäuser, Paulus and Krach2015; Reference Müller-Pinzler, Rademacher, Paulus and Krach2016; Paulus et al. Reference Paulus, Müller-Pinzler, Jansen, Gazzola and Krach2015). We propose that sleep strengthens and consolidates these single and emotionally laden encounters and idiosyncrasies in episodic memory.

Yet, in the long run it may not be efficient to remember every single encounter and every single detail in episodic memory (e.g., where it was, when it was, what happened). What is important is to remember the essential gist of the single encounters, which can then be stored in the form of new gist knowledge for the long term. Sleep may help abstract this gist from single social interactions, essentially building more general beliefs about others (e.g., this person is trustworthy). Social information about other people is particularly relevant in humans, considering that humans are highly social animals and social information about others can be critical for future situations. For example, remembering from current encounters whether another person is trustworthy is highly important for future social interactions when an individual has to decide whether or not to trust that person. In the worst case, this knowledge can decide about life or death. This critical importance of social information may also be signaled by the emotional salience of social encounters. By favoring such relevant and emotionally salient information, sleep may preferentially boost the consolidation and abstraction of social episodic memories over non-social memories to ensure the individual's social fitness.

In this way, sleep-dependent episodic memory consolidation may help decide what to believe about others and how to behave in future social interactions. Social episodic memories, having been processed, abstracted, and shaped during sleep, may give reasons to support social relationships and interpersonal communication. This social function may indeed be one of the most important functions of episodic memory consolidation during sleep in humans. Disruptions of normal sleep patterns may hamper the processing and abstraction of social gist information, potentially leading to impairments in social interaction and communication, such as in autism spectrum disorders (Devnani & Hegde Reference Devnani and Hegde2015). Understanding the relationship between episodic memory, sleep, and social functioning will be an intriguing and promising target for future research.