Closed-loop system to enhance slow-wave activity

Recent evidence reports cognitive, metabolic, and sleep restoration benefits resulting from the enhancement of sleep slow-waves using auditory stimulation. To make this concept practical for a consumer user, we have developed an integrated wearable closed-loop electroencephalogram (EEG) based system to deliver auditory stimulation during sleep. The stimulation is in the form of 50-millisecond-long tones separated by a constant one-second inter-tone interval at a volume that is dynamically modulated such that louder tones are delivered when sleep is deeper. The system was tested in a study involving 28 participants (18F, 10M; 36.9 ± 7.3 years old) who used the system for ten nights (5 nights in the sham condition and 5 in the stimulation condition). Four nights in each condition were recorded at home and the fifth one in-lab. Modeling the effect of the stimulation on slow-wave activity (SWA) through a mixed effects model revealed significant SWA enhancement moderated by age. The analysis in two age groups defined by the median age of participants in the study confirmed significant SWA enhancement (+16.1%, p < 0.01) for the younger group and absence of effect on the older group. However, the older group received only a fraction (57%) of the stimulation compared to the younger group. Well-known changes in sleep architecture and EEG properties due to aging, appear to have influenced the amount of stimulation. The analysis of the stimulation timing in the EEG suggests an entrainment-like phenomenon where slow-waves align to the stimulation periodicity. In addition, enhancement of spindle power in the stimulation condition was found.