Relaxing words in sleep slows down cardiac activity

Researchers from the GIGA - Centre of Research Cyclotron at the University of Liege shed new light on brain-heart interactions during sleep. They discovered that the body reacts to the external world when sleeping, explaining how sensory input might affect sleep quality.

Researchers worked with the University of Fribourg in Switzerland to study if the body genuinely disconnects from the outside world while sleeping. To do so, they examined how the heartbeat varies when we hear different words while sleeping.

They found that relaxing words slowed down cardiac activity as a reflection of deeper sleep and in comparison to neutral words that did not have such a slowing effect. This discovery is presented in the Journal of Sleep Research and sheds new light on brain-heart interactions during sleep.

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 ULiege teamed up with colleagues from University of Fribourg to lead a previous study analysing 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 hypothesized that the brain also remains able to interpret sensory information in a way that makes our body more relaxed after hearing relaxing words during sleep.

In this new study, the authors had the opportunity to analyse cardiac activity (electrocardiogram) to test this hypothesis and found that the heart slows down its activity only after the presentation of relaxing, but not control words.

Markers of both 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 directly contribute to the way we perceive the world, but such evidence has so far been obtained in wakefulness.

With these results, the ULiege 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", said 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", explained 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", Koroma advocated.