Slow Wave Entrainment Using a Smartwatch: A Randomized Crossover Study

BackgroundSlow-wave sleep is critical for sleep quality, cognitive function, and mood. Slow-wave entrainment (SWE) via rhythmic sensory stimulation can enhance slow-wave activity. However, existing implementations rely on EEG systems, thereby limiting accessibility and scalability. Consumer smartwatches offer an opportunity to deliver SWE in home settings without EEG hardware. ObjectiveThis study evaluated whether smartwatch-delivered sensory stimulation applied during smartwatch-estimated deep sleep elicits acute changes in frontal slow-wave EEG activity during home sleep, and whether individual differences in neural responsiveness to stimulation are associated with next-day behavioral and sleep measures. MethodsIn a randomized crossover design, participants recruited offline from the Boston area slept at home for two nights while wearing a consumer smartwatch for stimulation delivery and a portable EEG headband for neural recording. On a single night, participants received block-wise auditory, vibrotactile, or combined stimulation, guided by an automated on-watch sleep-staging model based on heart rate and motion. On the other night, no stimulation was delivered. Event-related changes in frontal delta (1-4 Hz) power were quantified relative to pre-stimulation baselines. Sleep disruption, subjective sleep quality, mood, and cognitive performance were assessed using questionnaires and a computerized Trail Making Test emailed to participants and completed online. ResultsInitiation of sensory stimulation was associated with significant increases in frontal delta power relative to pre-stimulation baseline and matched non-stimulation blocks.Stimulation blocks exhibited lower disruption rates than non-stimulation blocks, suggesting improved sleep stability during stimulation periods. No significant group-level differences were observed between stimulation and non-stimulation nights on measures of sleep quality, mood, or cognition. However, across participants, larger stimulation-evoked increases in delta power were associated with more favorable next-day subjective sleep and mood ratings and fewer clicks to complete the Trail Making Test. 68/93 participants were stimulated overnight. ConclusionsSmartwatch-based slow-wave entrainment delivered during home sleep can elicit reproducible delta EEG responses without sleep disruption. Individual differences in neural responsiveness to stimulation were associated with next-day behavioral measures, suggesting that wearable-based SWE may represent a scalable and accessible approach for improving sleep health. Trial RegistrationThe experiment was retrospectively registered at ISRCTN (registration number pending)