Beschreibung:
Abstract Background Research suggests that exposure therapy delivered in the morning is more successful than delivered in the evening, which is often explained by higher diurnal endogenous cortisol levels. However, this “morning exposure effect” might also be explained by other factors such as sleep or vigilance. Methods The current study aimed to disentangle these effects by assessing the impact of video-based exposure therapy delivered in the morning or in the evening, whilst considering pre-exposure sleep quality, vigilance, and cortisol levels. To this end, 80 snake fearful individuals were randomly assigned to receive exposure treatment in the morning or evening. Results Contrary to previous findings, groups did not differ in their pre-post and post-follow up decrease of snake anxiety. However, higher vigilance was found to be associated with a greater pre-post and post-follow-up decrease in snake anxiety. Moreover, pre-exposure sleep efficiency moderated the post-follow-up decrease in snake anxiety across groups: In individuals with high pre-exposure sleep efficiency, those receiving exposure in the morning were estimated to show a stronger decrease in snake anxiety than those receiving exposure in the evening. The opposite pattern was found in individuals with low pre-exposure sleep efficiency. Conclusions The results of this study illustrate that diurnal effects on exposure therapy might be more complex than previously assumed. Trial Registration The study was prospectively preregistered at the German Clinical Trial Register (https://drks.de/search/en/trial/DRKS00016183).