“We were expecting that stimulation with tones during deep sleep would impact 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 Neural Control of Movement Lab at ETH Zurich.
Heart specialist Schmied is also delighted: “We clearly saw that both the heart’s pumping force and its relaxation were greater after nights with stimulation compared to nights without stimulation.” Both factors are an excellent measure of cardiovascular system function.
Stimulation with pink noise
The study involved 18 healthy men aged 30 to 57, who spent three non-consecutive nights in the sleep laboratory. On two nights, the researchers stimulated the subjects with sounds; on one night, they did not.
While the subjects slept, the scientists continuously measured their brain activity, blood pressure and heart activity. They coupled their measurements to a computer system that analysed the incoming data.
As soon as the readings indicated that the subject had fallen into deep sleep, the computer played a series of very brief tones at certain frequencies, called pink noise, which sound like static. Ten seconds of such tones were followed by 10 seconds of silence, and then the same procedure could be repeated. A feedback mechanism ensured that the noise was played at the right time and – depending on the brain wave pattern – stopped again.
This experimental setup allowed the researchers to directly monitor whether the sound simulation enhanced deep sleep and whether it influenced the subjects’ heart rate and blood pressure. “During stimulation, we clearly see an increase in slow waves, as well as a response from the cardiovascular system that is reminiscent of cardiovascular pulsation,” says lead author Stephanie Huwiler, describing the direct effects during sleep.
The next morning, the heart specialists examined the subjects’ cardiac function using echocardiography (ultrasound).
Significant results despite small group
“Despite the relatively small group of subjects, the results are significant. We were also able to reproduce the results on two separate nights, which in statistical terms makes them very strong,” Lustenberger says.