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A group of researchers directly monitored whether sound simulation improved deep sleep and how it influenced people’s heart rate and blood pressure.
The human brain recovers during the deep sleep stage and the rest of the body appears to regenerate as well. Recently, researchers from ETH Zurich and the University of Zurich have shown that increased deep sleep is especially beneficial for the cardiovascular system.
They base these results on the fact that specific stimulation with brief tones during deep sleep causes the heart, particularly the left ventricle, to contract and relax more vigorously.
As a result, it pumps blood into the circulatory system and removes it more efficiently. The left ventricle supplies oxygen-rich arterial blood to most organs, the extremities, and the brain.
When the heart contracts, the left ventricle squeezes and wriggles out like a wet sponge. The more immediate and powerful this draining action is, the more blood enters the circulation and the less remains in the heart. This improves blood circulation, which is good for the heart.
An interdisciplinary team of heart specialists led by Christian Schmied, senior consultant for cardiology at University Hospital Zurich, used echocardiography (cardiac ultrasound examinations) to demonstrate that the left ventricle undergoes more intense deformation after nocturnal stimulation.
This is the first time anyone has shown that increased brain waves during deep sleep (slow waves) improve heart function. This study was published in the latest edition of the European Heart Journal.
“We expected that tone stimulation during deep sleep would affect the cardiovascular system. But the fact that this effect was so clearly measurable after just one night of stimulation surprised us,” explains project leader and sleep expert Caroline Lustenberger, SNSF Ambizione Fellow at the Laboratory for Neural Control of Movement at ETH Zurich.
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Heart specialist Schmied is also delighted: “We clearly saw that both the pumping force of the heart and its relaxation were greater after nights with stimulation than during nights without stimulation.” Both factors are an excellent measure of the function of the cardiovascular system.
Sleep stimulation through sound
The study involved 18 healthy men between 30 and 57 years old, who spent three non-consecutive nights in the sleep laboratory. For two nights, the researchers stimulated the subjects with sounds; one night they didn’t.
While the participants were asleep, the researchers monitored brain, blood, and heart activity at regular intervals. They connected their measurements to a computer system that analyzed the incoming data.
As soon as the readings indicated that the subject had fallen into a deep sleep, the computer played a series of very brief tones at certain frequencies, called pink noise, that sounded like static.
The ScitechDaily review explains that ten seconds of those tones were followed by 10 seconds of silence, and then the same procedure could be repeated. The feedback system made sure that the noise played at the correct time and paused depending on the pattern of the brain waves.
This experimental setup allowed the researchers to directly monitor whether the sound simulation improved deep sleep and influenced the subjects’ heart rate and blood pressure.
“During stimulation, we clearly see an increase in slow waves, as well as a response of the cardiovascular system reminiscent of cardiovascular pulsation,” says lead author Stephanie Huwiler, describing the direct effects during sleep.
The next morning, cardiologists examined the subjects’ heart function using echocardiography (ultrasound).
