Auditory stimulation enhances brain activity during sleep in older adults

Though auditory stimulation can effectively and remotely improve slow wave activity during sleep in older adults, its effects vary greatly between different individuals, according to a recent study.

“Novel personalization solutions are needed to address these differences and our findings will guide future designs to effectively deliver auditory sleep stimulations using wearable technology,” the researchers said.

The present randomized crossover trial was conducted fully remotely, enrolling 16 healthy older adults between the ages of 62 and 78 years. Auditory stimulation, the intervention of interest, was delivered through a wearable device, as controlled by a mobile phone, and lasted for 2 weeks.

During the intervention phase, researchers assessed outcomes, including slow wave activity, sleep architecture, and daily functions, and compared these against findings during a 2-week sham period. The intervention and sham periods were interleaved by a 2-week washout period.

Electroencephalography revealed that during nonrapid eye movement sleep, auditory stimulation induced a significant to trend-level increase in spectral power in the low-frequency slow wave range, as compared with the sham arm. Such an effect showed strong dose dependency. [Commun Med 2022;2:30]

Indeed, robust linear mixed modelling showed that an increasing number of stimulations led to a more pronounced effect on slow wave activity (p=3.19×10^–5). There was also a significant main effect of the active intervention on increasing low-frequency slow wave activity overall (p=9.97×10^–4), with the magnitude of its impact being stronger during windows containing more auditory stimuli.

Exploratory post hoc analysis revealed that when only one to two auditory stimuli were delivered, the wearable device was not significantly different to sham. However, at ≥2 stimulations, low-frequency slow wave activity was significantly better than in the sham arm (3–4 stimulations: p=0.01; >4 stimulations: p=0.017).

Individualized response, personalized intervention

However, the researchers observed that there was a large variability in the interindividual differences in response to auditory stimulations.

Within a single night, for example, some participants might response strongly to the device and show large differences in low-frequency slow wave activity as compared with sham. At the same time, others might not benefit much from auditory stimulation, with their slow wave activity readings overlapping robustly between the active and sham treatments.

Typically, strong responders were significantly younger (mean, 65.5 vs 73.5 years; p=7.8×10^–5) and had higher baseline slow wave activity than weak responders.

“Sleep’s restorative function is closely linked to slow waves, which are brain activity patterns that occur during deep sleep and are diminished with age,” the researchers explained. “Those slow waves can be increased through auditory stimulation, a method that administers precisely timed sounds during sleep.”

The present study, they continued, demonstrated that with the help of a wearable device and mobile phones, it is feasible to deploy such an intervention to a population that needs it even in an uncontrolled setting.

Nevertheless, the substantial interindividual variabilities in response “highlight the importance of conducting future large-scale studies to establish secondary effects across time and diverse populations,” the researchers said.

“Our study provides means to optimize the rational design of auditory deep sleep stimulation and represents a crucial step towards successful and safe in-home implementation for prevention and therapy in the general population,” they added.