Enhancing Deep Sleep

Summary: SleepLoop, a newly designed mobile system, can help promote deep sleep through auditory brain stimulation. The device tracks when a person enters slow-wave brain activity, triggering an auditory signal that helps synchronize neurons and enhance the slow waves.

Source: ETH Zurich

Many people, especially the elderly, suffer from abnormal sleep. In particular, the deep sleep phases become shorter and shallower with age. Deep sleep is important for the regeneration of the brain and memory, and also has a positive influence on the cardiovascular system.

Researchers have shown that the brain waves characterizing deep sleep, so-​called slow waves, can be improved by playing precisely timed sounds through earphones while sleeping. While this works well in the sleep laboratory under controlled conditions, there has thus far been no at home solution that can be used longer than just one night.

SleepLoop to the rescue

As part of the SleepLoop project, researchers at ETH Zurich have developed a mobile system that can be used at home and aims to promote deep sleep through auditory brain stimulation.

The SleepLoop system consists of a headband that is put on at bedtime and worn throughout the night. This headband contains electrodes and a microchip that constantly measure the brain activity of the person sleeping. Data from this is analysed autonomously in real-​time on the microchip using custom software.

As soon as the sleeping person shows slow waves in the brain activity characterising deep sleep, the system triggers a short auditory signal (clicking). This helps to synchronise the neuronal cells and enhance the slow waves. What makes the solution unique is that the person sleeping is not conciously aware of this sound during deep sleep.

The first clinical study

Researchers from ETH Zurich and University Hospital Zurich, led by Caroline Lustenberger, group leader at the Neural Control of Movement Lab, have conducted a clinical study with this device for the first time. The results have just been published in the journal Communications Medicine.

The study involved equipping participants, between 60 – 80 years old, with the SleepLoop system, which they were required to operate themselves in their own home. The system is designed to function independently even by users with little technical experience. “This worked very well. We had surprisingly little data loss and the participants rated the device as user-​friendly,” says Lustenberger.

They wore the device every night for a total of four weeks, with the auditory stimulation given on a nightly basis for two weeks and no stimulation for the next two weeks. Neither the subjects nor the researchers knew in which two weeks the auditory signals were played and in which two they were not.

Auditory stimulation is indeed feasible

The results of 16 participants of the study show that it was indeed possible to enhance the slow waves through auditory signals during deep sleep in most participants. However, the individual differences were extensive with some of the subjects responding very well to the stimuli, while others responded minimally or not at all.

This shows a man sleeping with the SleepLoop system on his head
The SleepLoop system developed by ETH Zurich researchers emits a sound at the right time to amplify the slow brain waves. Credit: SleepLoop

According to Lustenberger, the question of whether a person reacted to a stimulus did not depend on their well-​being during the day. “Some people generally responded well to the stimuli and clearly showed enhanced slow waves, while others showed no response, regardless of their daily well-​being.”

The researchers have used these individual differences to better predict how a given individual will respond to the auditory stimulus. This in turn helps them to optimise and improve the performance of SleepLoop.

On track for market launch

A spin-​off company Tosoo AG, is currently working on developing the device further and preparing it for the clinical market. It is already clear that it will not be freely available, but only via a doctor’s prescription.

“This is a medical device, not just a wellness consumer product you can order online when you have trouble sleeping,” emphasises Walter Karlen, who developed the technology at ETH Zurich. 1 Karlen has been appointed Director of the Institute of Biomedical Engineering at Ulm University in May 2021.

“Use of the device must be medically indicated and supervised by a doctor”, he says. Further development of the technology will now continue also in Ulm.

About this neurotech and sleep research news

Author: Peter Rueegg
Source: ETH Zurich
Contact: Peter Rueegg – ETH Zurich
Image: The image is credited to SleepLoop

Original Research: Open access.
Auditory deep sleep stimulation in older adults at home: a randomized crossover trial” by Lustenberger C et al. Communications Medicine


Auditory deep sleep stimulation in older adults at home: a randomized crossover trial


Auditory stimulation has emerged as a promising tool to enhance non-invasively sleep slow waves, deep sleep brain oscillations that are tightly linked to sleep restoration and are diminished with age. While auditory stimulation showed a beneficial effect in lab-based studies, it remains unclear whether this stimulation approach could translate to real-life settings.


We present a fully remote, randomized, cross-over trial in healthy adults aged 62–78 years (clinicaltrials.gov: NCT03420677). We assessed slow wave activity as the primary outcome and sleep architecture and daily functions, e.g., vigilance and mood as secondary outcomes, after a two-week mobile auditory slow wave stimulation period and a two-week Sham period, interleaved with a two-week washout period. Participants were randomized in terms of which intervention condition will take place first using a blocked design to guarantee balance. Participants and experimenters performing the assessments were blinded to the condition.


Out of 33 enrolled and screened participants, we report data of 16 participants that received identical intervention. We demonstrate a robust and significant enhancement of slow wave activity on the group-level based on two different auditory stimulation approaches with minor effects on sleep architecture and daily functions. We further highlight the existence of pronounced inter- and intra-individual differences in the slow wave response to auditory stimulation and establish predictions thereof.


While slow wave enhancement in healthy older adults is possible in fully remote settings, pronounced inter-individual differences in the response to auditory stimulation exist. 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.

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