Effect of frontal 30-Hz-tACS on subjective and objective sleepiness.

Transcranial Current stimulations (tCSs) are non-invasive brain stimulation techniques which are able to modulate cortical excitability and spontaneous brain activity in a safe and quite predictive manner. In tACS protocols, the current is time-varying and the stimulation interfers with the spontaneous brain oscillations, according to the principle of entrainment. Previous studies showed that frontal-5-Hz-tDCS is able to induce an increase in subjective sleepiness associated to an enhancement of the delta EEG activity. Here, we evaluated the ability of high-frequency-tACS to modulate sleepiness in the opposite direction, i.e. to reduce sleepiness and slow-frequency activity. Ten subjects underwent a within-subject study (Active/Sham), with 30-min session (10-min pre-stimulation EEG; 10-min 30-Hz-tACS; 10-min post-stimulation EEG). During the whole protocol subjects were asked to stay relaxed with eyes closed. Self-reported sleepiness measures were collected before and after each stimulating session. The active stimulation was associated to decreased sleepiness in the post-stimulation interval, while the sham stimulation showed an increase in sleepiness scores. This pattern was coherent with the EEG data, with respect to the delta and gamma bands. In fact, compared to the sham condition, the active stimulation resulted in smaller increases of slow-frequency activity and in an increase of high-frequency activity. These results show that frontal-30-Hz-tACS is able to contrast the spontaneous increase of sleepiness during resting-state. This sleep-preventing effect, as well as our previous findings, opens up fascinating perspectives for a bidirectional manipulation of sleepiness by tCSs, with possible applications in basic and clinical sleep research.