SleepLoop can help promote deep sleep through auditory brain stimulation


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.

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.

SleepLoop is a wearable sleep monitoring and stimulation system developed at the Universities of Zurich (ETH Zurich and University of Zurich). The aim of SleepLoop is to enhance health by using mobile technologies that improve and deepen sleep in various stages of life from childhood to old age.  SleepLoop enables large scale research outside of the lab and will support all that need deeper and a more regenerative sleep.  
SleepLoop has been awarded the Hochschulmedizin Zurich Flagship Award in 2017.

Why healthy sleep is important

Sleep is essential for life, yet, we do not sleep enough! A multitude of conditions have been linked to insufficient sleep. Reduced vigilance and decreased performance levels are the most prominent. In addition, chronic lack of sleep has been associated with metabolic diseases such as diabetes as well as brain disorders such as Alzheimer’s disease. 

Today, it is fairly easy to observe and/or diagnose insufficient sleep. However, only one third of chronically sleep restricted humans can comply with longer sleep times. High work burden, a multitude of social commitments, the use of electronic devices and other factors prevent most people from complying with healthy sleep habits. Medication to promote and deepen sleep is ineffective due to tolerability and dependence issues. To date, there was no efficient strategy for long-term improvement of sleep.

How does SleepLoop work?

At night, a wearable device records brain activity. During deep sleep where neurons are synchronized, a stimulus is provided in form of an auditive tone. By playing this tone at the right moment in time, synchronization of neurons can be enhanced and the person can sleep even deeper, which ultimately leads to a higher quality of sleep.

How will people benefit from SleepLoop?

SleepLoop is used to investigate a multitude of aspects such as

  • General brain function
  • Sleeping disorders
  • Depression
  • Epilepsy
  • Traumatic brain injury
  • Behavioral economics
  • Parkinson’s disease

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


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