Researchers studied whether the body genuinely disconnects from the outside world during sleep. To do so, they studied how our heartbeats vary when we hear different words while sleeping. They discovered that calming words reduced cardiac activity in response to deeper sleep, as opposed to neutral phrases, which did not have the same effect. This discovery sheds light on how the brain and heart interact when sleeping.
Researchers from the GIGA — Center of Research Cyclotron at the University of Liège have discovered that the sleeping body reacts to the external world while sleeping, which explains how some sensory input might influence sleep quality.
Researchers from ULiège worked with the University of Fribourg in Switzerland to study if the body genuinely disconnects from the outside world while sleeping. They focused on how the heartbeat changes when we hear different words while sleeping. They discovered that calming words reduced cardiac activity as a reflection of deeper sleep, in contrast to neutral phrases, which did not have the same effect. This result, reported in the Journal of Sleep Research, throws fresh light on brain-heart interactions during sleep.
Most of the sleep research focuses on the brain and rarely investigates bodily activity. We nevertheless hypothesize that the brain and the body are connected even when we cannot fully communicate, including sleep.
Dr. Schmidt
Matthieu Koroma (Fund for Scientific Research — FNRS postdoctoral researcher), Christina Schmidt, and Athena Demertzi (both Fund for Scientific Research — FNRS Research Associate) from the GIGA Cyclotron Research Center at ULiège teamed up with colleagues from the University of Fribourg led a previous study analyzing brain data (electroencephalogram) showing that relaxing words increased deep sleep duration and sleep quality, showing that we can positively influence sleep using meaningful words.
By that time, the authors reasoned that the brain might still process sensory input in a way that made our bodies feel more relaxed after hearing soothing phrases while sleeping. In this new study, the authors were able to measure cardiac activity (electrocardiogram) to test this idea and discovered that the heart slows down only after the display of calming, not control, phrases.
Markers of cardiac and brain activity were then compared to disentangle how much they contributed to the modulation of sleep by auditory information. Cardiac activity has been indeed proposed to contribute to the way we perceive the world directly, but such evidence has so far been obtained in wakefulness. With these results, the ULiège researchers showed that it was also true in sleep, offering a new perspective on the essential role of bodily reactions beyond brain data for our understanding of sleep.
“Most of sleep research focuses on the brain and rarely investigates bodily activity,” says Dr. Schmidt.
“We nevertheless hypothesize that the brain and the body are connected even when we cannot fully communicate, including sleep. Both brain and body information need then to be taken into account for a full understanding of how we think and react to our environment,” explains Dr. Demertzi.
“We shared freely our methodology following the principles of Open Science hoping that the tools that helped to make this discovery will inspire other researchers to study the role played by the heart in other sleep functions,” Dr. Koroma said.
This study takes a more complete approach to the modulation of sleep functions by sensory information. By investigating cardiac reactions to noises, we may be able to research in the future the involvement of the body in the way sounds influence emotional processing of memories while sleeping.
