We thank Dr Dardennes and colleagues (Dardennes et al. Reference Dardennes, Alanbar, Docteur, Divac and Mirabel-Sarron2014) for their thoughtful and informed comments and their interest in our study in which we have reported that sleep after exposure therapy improves therapeutic success in individuals with spider phobia (Kleim et al. Reference Kleim, Wilhelm, Temp, Margraf, Wiederhold and Rasch2014). In our paper, we suggest that the beneficial effect of sleep on exposure therapy might be explained by an active role of sleep for the consolidation of extinction memories (Rasch & Born, Reference Rasch and Born2013). According to Dardennes and colleagues, the beneficial effect of sleep on exposure therapy outcome could alternatively be explained by the role of sleep in avoiding reactivation of fear memories which might occur during post-therapy wakefulness.

More specifically, Dardennes and colleagues suggest that in our study, spider-related cues might have induced reactivation and reconsolidation of fear memories during film-viewing of our wake control group. We consider this possibility unlikely, because we carefully selected and pre-screened the movie to ensure absence of obvious spider cues. Furthermore, a post-film questionnaire revealed no evidence for any disturbing or distracting elements in the movie. Importantly, none of our participants reported anything related to spiders or spider-related cues. Thus, the movie clearly contained no obvious or consciously perceived spider cues.

However, we cannot exclude that some non-obvious cues might have subconsciously reactivated fear memories during wakefulness, which could happen in any environment independent from the movie we used in our study. Even in the case of reducing external input to a minimum (e.g. staying awake in a dark room), internally triggered reactivation of fear memories might occur. Thus, wakefulness per se might be a state with a higher chance of externally or internally triggered reactivation and reconsolidation of fear memories, possibly hindering successful consolidation of extinction memories. It is important to note that sleep is the only naturally occurring (non-pharmacologically induced) state in which external and internal interference is strongly reduced, which might indeed contribute to the beneficial effect of sleep for memory consolidation (see Mednick et al. Reference Mednick, Cai, Shuman, Anagnostaras and Wixted2011).

Identifying an active contribution of sleep after exposure therapy might require more sophisticated experimental designs. For examples, night-half designs comparing memory consolidation during early, SWS rich sleep v. late REM sleep have successfully revealed differential contributions on neutral v. emotional memories (e.g. Plihal & Born, Reference Plihal and Born1997; Wagner et al. Reference Wagner, Gais and Born2001, Groch et al. Reference Groch, Wilhelm, Diekelmann and Born2013). Furthermore, experimentally inducing slow oscillations (Marshall et al. Reference Marshall, Helgadottir, Mölle and Born2006) or cueing memories during SWS (Rasch et al. Reference Rasch, Buchel, Gais and Born2007) improves consolidation processes during sleep, which could be successfully adapted to further improve the beneficial effect of sleep on therapy outcome. In our study, the finding that greater percentages of stage 2 sleep were associated with greater reductions in fear and negative cognitions also speaks in favour of active consolidation processes acting during sleep. Further studies are certainly needed in order to explore and disentangle active mechanisms that contribute to the beneficial effects of sleep on emotional learning during therapy.