Double-blind repeated measures tACS stimulation (30 s duration, each) in REM sleep over 4 non-consecutive nights. Frequency of stimulation (sham, 2, 6, 12, 25, 40, 70, and 100 Hz) was counterbalanced across subjects and across nights. The experimenter operating the tACS device did not interact with the subjects. The experimenter conducting the interview stayed outside the monitoring room during stimulation, unable to identify the stimulation condition (for sham stimulation, the push button on the tACS device was activated but current was not applied). Extensive exploration gave no indication of subjective discomfort or awareness of stimulation in any subject.