Chronotype in alpha-tACS: Preliminary evidence hints at sleep quality modulation of aftereffects in evening types in the morning.

Transcranial alternating current stimulation (tACS) is a promising tool for research on oscillatory brain activity, yet both behavioral and electrophysiological outcome measures show high variability across studies. One source for this variability might be chronotype and an incidental mismatch between chronotype and the time of the measurement. 14 evening type and 14 morning type participants performed a sustained attention task - once at their chronotypically optimal and once at a non-optimal time of day. TACS was applied for 20 min at the individual alpha frequency over two electrodes located at Cz and Oz. EEG was recorded for 10 min prior to and after stimulation. Sleep timing and quality were assessed with a sleep questionnaire. While planned analyses failed to find effects of stimulation and session timing on alpha power, exploratory analyses revealed that below average sleep quality in evening types in the morning was associated with no changes or unexpected decreases in alpha power after stimulation. Effects of sleep quality were present in the morning for evening types, but neither in the evening session nor in morning types. It is suggested that this effect of sleep quality reflects increased sleepiness, which could impede expected aftereffects of tACS. It is likely that effects of sleepiness might be especially relevant when people are stimulated at a chronotypically non-optimal time. Due to the exploratory nature of these sleep effects and their presence in only a small subgroup leading to low power and confidence, future systematic sham-controlled studies are needed to clarify the relationship between sleep, time of day and chronotype in