We greatly appreciate the target article by Lane et al. highlighting the importance of recent findings in the brain sciences for understanding and improving the mechanisms of action in psychotherapy. We believe that it is high time to incorporate this knowledge into psychotherapy research, as well as into practical psychotherapy and education. Lane et al. discuss compellingly the role of maladaptive emotional memories in psychopathology and the possibility to change dysfunctional memories through new corrective experiences in the therapy setting via processes of reconsolidation. Although we are in perfect agreement with this account, we want to highlight a potentially crucial factor in this process: the functional role of sleep.

Sleep is well-known to enhance the consolidation of freshly acquired memories, particularly emotional memories (Payne & Kensinger Reference Payne and Kensinger2010; Rasch & Born Reference Rasch and Born2013; Stickgold & Walker Reference Stickgold and Walker2013). Delayed memory retrieval is typically enhanced if the initial acquisition of new memories is followed by a period of sleep compared with an equivalent wake period, with sleep occurring shortly after learning being more effective than delayed sleep (Gais et al. Reference Gais, Lucas and Born2006). Some forms of memory even require sleep during the first night after learning, with the new memory being entirely lost if sleep is forgone (Stickgold et al. Reference Stickgold, James and Hobson2000). For many forms of memory, brief naps of 40 to 90 minutes are sufficient to promote consolidation processes (Diekelmann et al. Reference Diekelmann, Biggel, Rasch and Born2012; Mednick et al. Reference Mednick, Nakayama and Stickgold2003; Tucker et al. Reference Tucker, Hirota, Wamsley, Lau, Chaklader and Fishbein2006). One study suggests that even a very short nap of only 6 minutes can improve memory performance even though longer naps provide stronger improvements (Lahl et al. Reference Lahl, Wispel, Willigens and Pietrowsky2008).

Apart from the strengthening and stabilization of memories, sleep also facilitates the integration of new memories into pre-existing schemas and semantic networks (Ellenbogen et al. Reference Ellenbogen, Hu, Payne, Titone and Walker2007; Landmann et al. Reference Landmann, Kuhn, Piosczyk, Feige, Baglioni, Spiegelhalder, Frase, Riemann, Sterr and Nissen2014; Tamminen et al. Reference Tamminen, Lambon Ralph and Lewis2013), a function that seems to be of particular relevance in the context of changing and updating memories in psychotherapy. Reconsolidation of memories after reactivation during wakefulness (e.g., via retrieval) has likewise been suggested to benefit from sleep (Walker et al. Reference Walker, Brakefield, Hobson and Stickgold2003). It is generally believed that the consolidating function of sleep for memory relies on the neuronal reactivation (“replay”) of new in conjunction with older memory representations during sleep, possibly in concurrence with a selective downscaling process, such that the respective memories are stronger and better integrated after sleep (Diekelmann & Born Reference Diekelmann and Born2010; Lewis & Durrant Reference Lewis and Durrant2011; Tononi & Cirelli Reference Tononi and Cirelli2014).

Apart from this memory-improving effect of normal sleep, recent studies suggest that specific characteristics of sleep can be directly targeted to enhance sleep's beneficial effect (Diekelmann Reference Diekelmann2014; Spiers & Bendor Reference Spiers and Bendor2014). For example, facilitating memory reactivation by presenting olfactory or auditory cues during sleep that have previously been associated with the learning experience enhances memory consolidation (Oudiette & Paller Reference Oudiette and Paller2013; Rasch et al. Reference Rasch, Buchel, Gais and Born2007). Such targeted memory reactivations can specifically enhance those memories that are cued during sleep while leaving uncued memories unaffected (Rudoy et al. Reference Rudoy, Voss, Westerberg and Paller2009; Schonauer et al. Reference Schonauer, Geisler and Gais2014). Re-exposure of olfactory context cues during sleep that had been present during prior fear conditioning might even induce extinction of the conditioned fear response (Hauner et al. Reference Hauner, Howard, Zelano and Gottfried2013; but see Barnes & Wilson Reference Barnes and Wilson2014; Rolls et al. Reference Rolls, Makam, Kroeger, Colas, de Lecea and Heller2013).

Increasing slow oscillations (<1 Hz, the hallmark brain oscillation of slow wave sleep) by electrical transcranial direct current stimulation (tDCS) or auditory stimulation is another promising method to enhance sleep-dependent memory processing (Marshall et al. Reference Marshall, Helgadottir, Molle and Born2006; Ngo et al. Reference Ngo, Martinetz, Born and Molle2013). Applying electrical currents that oscillate at the same frequency as natural slow oscillations intensifies endogenous slow oscillations and improves memory consolidation (Marshall et al. Reference Marshall, Helgadottir, Molle and Born2006). Similar increases in slow oscillations and associated memory performance are observed following timed auditory stimulation of slow oscillations (Ngo et al. Reference Ngo, Martinetz, Born and Molle2013). A third way to manipulate sleep and memory relates to pharmacological interventions. Several drugs targeting different neurotransmitter systems have been proven effective to enhance memory during sleep, such as drugs manipulating neurotransmission of noradrenaline (Gais et al. Reference Gais, Rasch, Dahmen, Sara and Born2011), dopamine (Feld et al. Reference Feld, Besedovsky, Kaida, Münte and Born2014), glutamate (Feld et al. Reference Feld, Lange, Gais and Born2013), and GABA (Kaestner et al. Reference Kaestner, Wixted and Mednick2013).

Many psychiatric disorders are associated with impaired sleep and memory dysfunctions, such as post-traumatic stress disorder (PTSD) (Germain Reference Germain2013), depression (Steiger et al. Reference Steiger, Dresler, Kluge and Schussler2013), and schizophrenia (Lu & Goder Reference Lu and Goder2012). Improving sleep in these patients might generally ameliorate disorder-related symptoms and improve cognitive performance. Patients with schizophrenia, for example, show reduced sleep-dependent memory consolidation (Goder et al. Reference Goder, Boigs, Braun, Friege, Fritzer, Aldenhoff and Hinze-Selch2004), while electrical slow oscillation stimulation during sleep increases memory functions in these patients (Goder et al. Reference Goder, Baier, Beith, Baecker, Seeck-Hirschner, Junghanns and Marshall2013). Apart from a generally positive effect of restoring normal sleep patterns, we want to suggest that sleep can specifically support the strengthening and integration of emotional memories that have been updated during prior psychotherapy. Two recent studies provide first evidence that sleep after exposure therapy improves therapy outcome in spider phobia (Kleim et al. 2013; Pace-Schott et al. Reference Pace-Schott, Verga, Bennett and Spencer2012). Patients underwent a virtual reality exposure session and were allowed to sleep for 90 minutes after the treatment (Kleim et al. 2013). At a follow-up test one week later, these patients reported significantly reduced fear and spider-related cognitions compared with a group of patients that had stayed awake after the treatment. It remains to be elucidated whether targeted sleep manipulations, such as cued memory reactivation and slow oscillation stimulation, can boost this effect further.

Based on this evidence, we suggest that sleep and specific sleep interventions can facilitate memory updating and thereby improve therapy gain in memory-related psychopathology. Future research should test whether certain sleep interventions are more effective for certain types of psychotherapy and how sleep interventions can best be incorporated into the therapy setting to optimize outcome. We believe that sleep interventions are highly promising new therapy tools as they do not only bear a strong potential to increase therapy success but at the same time are cost-effective and have no to little side effects.