Beyond Hypnograms: Assessing Sleep Stability Using Acoustic and Electrical Stimulation.

OBJECTIVE: Conventional polysomnographic recordings reflect brain dynamics associated with sleep architecture. We hypothesized that noninvasive tools like transcranial alternating current stimulation (tACS) and acoustic stimulation (for generating event related potentials [ERPs]) would help to predict sleep stability and provide a window to actively assess brain activity during sleep.

MATERIALS AND METHODS: Twelve healthy male volunteers participated in the multiple whole-night polysomnography (PSG) recording protocol. Acoustic tones (100 msec duration) were presented throughout night to evaluate ERP during sleep. Furthermore, 30 sec tACS were presented during nonrapid eye movement (NREM) and rapid eye movement (REM) sleep on subsequent two nights without disturbing the subject's sleep. For ERP analysis, event-locked artifact-free epochs from each sleep stage were averaged separately. For tACS analysis, 30 sec prestimulus and poststimulus artifact-free EEG epochs were subjected to bootstrapping-based comparison of power spectral values.

RESULTS: Acoustic stimulation generated sleep stage-dependent ERP components (N350, N550, and P900) in all participants. The tACS stimulation during NREM sleep (0.75 Hz) increased parietal delta power but decreased frontocentral theta and increased frontal gamma power when delivered during REM sleep (40Hz). These interventions provide details on sleep stability as larger N550-P900 ERP-complex correlated with lower NREM disruptions (Spearman's rho = -0.553; p = 0.049; n = 10) and tACS-related theta power perturbation with higher REM disruptions (Spearman's rho = 0.734; p = 0.030; n = 7).

CONCLUSIONS: Noninvasive brain stimulation approaches such as sleep ERP and sleep tACS are reliable tools to evaluate sleep stability during NREM and REM sleep, respectively, but more large-sample studies are warranted.