Anticipation of stressful events can impair sleep quality. In a recent study, we reported that anticipating a stressful task before a nap led to negative changes in sleep parameters, particularly at the end of the nap. In our previous study, we compared stress anticipation with the anticipation of relaxation; thus, the observed effects may have been amplified by sleep quality improvements in the relaxation condition. In the current study, we aimed to replicate these findings using an alternative neutral control condition. The data from a newly collected sample (n = 31) were compared with the data from our previous study (n = 33) using identical analyses. The results reveal an opposite pattern from our previous study: participants in the neutral control condition showed poorer sleep (longer sleep onset latency, reduced slow-wave sleep, and lower SWA/beta ratio) compared to those anticipating stress. In a direct comparison of both studies, sleep parameters in the stress conditions were highly similar across the two studies, suggesting that the divergent outcomes are driven by differences in the control conditions. The temporal dynamic changes observed in our previous study could not be replicated. These findings highlight the importance of carefully considering control conditions in experimental sleep research and suggest that even “neutral” instructions can evoke anticipatory effects. Moreover, the observed benefits of anticipating post-sleep relaxation highlight opportunities for relaxation-based interventions to improve sleep quality.

Targeted memory reactivation (TMR) is an effective technique to enhance sleep-associated memory consolidation. The successful reactivation of memories by external reminder cues is typically accompanied by an event-related increase in theta oscillations, preceding better memory recall after sleep. However, it remains unclear whether the increase in theta oscillations is a causal factor or an epiphenomenon of successful TMR. Here, we used transcranial alternating current stimulation (tACS) to examine the causal role of theta oscillations for TMR during non-rapid eye movement (non-REM) sleep. Thirty-seven healthy participants learned Dutch–German word pairs before sleep. During non-REM sleep, we applied either theta-tACS or control-tACS (23 Hz) in blocks (9 min) in a randomised order, according to a within-subject design. One group of participants received tACS coupled with TMR time-locked two seconds after the reminder cue (time-locked group). Another group received tACS in a continuous manner while TMR cues were presented (continuous group). Contrary to our predictions, we observed no frequency-specific benefit of theta-tACS coupled with TMR during sleep on memory performance, neither for continuous nor time-locked stimulation. In fact, both stimulation protocols blocked the TMR-induced memory benefits during sleep, resulting in no memory enhancement by TMR in both the theta and control conditions. No frequency-specific effect was found on the power analyses of the electroencephalogram. We conclude that tACS might have an unspecific blocking effect on memory benefits typically observed after TMR during non-REM sleep.

Pre-sleep cognitive activity is a well-known factor affecting sleep; however, the mechanisms underlying these effects remain unclear. The “Memories-of-Sleep” (MemoSleep) hypothesis proposes that pre-sleep wake- and sleep-related memories are reactivated during sleep, triggering associated neural and physiological responses and shaping sleep physiology and architecture. Dreams may be particularly informative in this context, having the potential to reflect ongoing reactivation processes during sleep. This dissertation aimed to provide empirical support for this reactivation mechanism, thereby advancing the understanding of the relationship between sleep and cognition. Four controlled laboratory studies were conducted, each investigating distinct cognitive influences on sleep and testing predictions derived from the MemoSleep framework. Together, the findings provide converging evidence for the proposed reactivation mechanism, while highlighting important temporal dynamics and motivating several extensions of the framework. Across studies, pre-sleep cognitive-emotional states were differentially reactivated during sleep, as reflected in dream characteristics and selective modulations of sleep physiology. Building on these results, an extended MemoSleep framework is outlined to advance the understanding of how cognitive-emotional material shapes reactivation processes and their consequences for sleep and dreams. By conceptualising cognitive activity during sleep as a multidimensional continuum, this extended framework enables the consideration of individual differences and informs the development of cognitive interventions to improve sleep quality. Overall, this dissertation provides a conceptual framework that enables future research on sleep-promoting cognitions and their interaction with physiological and subjective dimensions of sleep, while also situating dreams as a window into cognitive-emotional processes during sleep.