Effects of fronto-temporal 5 Hz-tACS on subjective and objective sleepiness

Recent findings showed that oscillating transcranial Direct Current Stimulation (osc-tDCS) and transcranial Alternating Current Stimulation (tACS) applied during wake and sleep can interact with the ongoing brain activity in a frequency-specific manner. Here, we evaluated the efficacy of a bilateral 5Hz-tACS on fronto-temporal areas in modulating physiological sleepiness during wakefulness, by inducing synchronization in cortical and subcortical structures involved in the sleep onset. Twenty-six healthy volunteers (18-35 years) participated in two within-subject sessions (Active and Sham), one week apart and in counterbalanced order. Each session involved: 5-min pre-stimulation EEG; 10-min tACS stimulation; 5-min post-stimulation EEG. The self-reported levels of sleepiness were collected with the Karolinska Sleepiness Scale (KSS) and Visual Analogue Scale (VAS) at the beginning and at the end of the two experimental sessions. A sinusoidal AC was applied (current intensity: 0.6 mA; max current density=0.531 mA/cm2) by two electrodes, located on FT7 and FT8. Power spectra of the 28 EEG derivations were computed by a fast Fourier transform routine in 2 s epochs across the EEG bands. Results show that 5Hz-tACS on fronto-temporal area induces an increase (10-15%) of EEG power in lower frequency bands (delta and theta) but fails to induce any effect on subjective sleepiness. The specific topographic pattern of changes involving frontal increase in the delta band and occipital increase in the theta range seems to simulate the dynamics characterizing an early sleep onset. However, no causal relation can be traced on the basis of the current results between these rhythms and changes on sleepiness.