Abstract
Transcranial direct current stimulation (tDCS) as a means for enhancing learning and memory has received a lot of attention in recent times. However, its applicability in a wider context has been limited due to lack of replicability across the literature. This may likely stem from inter-individual differences such as age, gender, nutrition, stress, brain morphology and sleep. Sleep in particular may be a source of inter-individual differences in tDCS-effect because of its link to brain plasticity mechanism such as long-term potentiation (LTP). The extent to which sleep may account for inter-individual differences in tDCS outcomes has not been assessed in the literature. Therefore, the central aim of this thesis is to investigate 1) the effect of sleep quality 2) circadian mis- /alignment 3) prior sleep compared to wake on tDCS-enhanced learning. Findings from this thesis suggests that sleep quality does not affect variability in tDCS-effect on cognitive performance, while circadian mis/-alignment and prior wakefulness before task may modulate tDCS-efficacy. In conclusion, data suggests that tDCS-effect is greater in a brain which is in a non-optimal state in terms of circadian misalignment and prolonged wake, and in this context, sleep may be responsible for variabilities in tDCS studies. These findings have implications for researchers and clinicians using tDCS. Further studies are required to fully characterise the findings from this thesis.