A discovery by researchers at the GIGA Cyclotron Research Center at the University of Liège reveals that the sleeping body also reacts to the external world during sleep, explaining how certain information from the sensory environment can affect sleep quality.
Researchers from ULiège have collaborated with the University of Freiburg (Switzerland) to investigate whether the body is truly disconnected from the outside world during sleep. To do this, they focused on how the heartbeat changes when we hear different words during sleep. They found that relaxing words slowed down heart activity as a reflection of deeper sleep and compared to neutral words that had no such slowing effect. This discovery is presented in Sleep Research Journal 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 ULiège teamed up with colleagues from the University of Freiburg and led previous research. A study analyzing brain data (electroencephalogram) shows that relaxing words increase the duration and quality of deep sleep, proving that we can positively influence sleep using meaningful words. At that time, the authors hypothesized that the brain is also still able to interpret sensory information in a way that makes our body more relaxed after listening to relaxing words during sleep. In this new study, the authors had the opportunity to analyze cardiac activity (electrocardiogram) to test this hypothesis and discovered that the heart slows down its activity only after the presentation of relaxing words, but not control words.
Markers of heart and brain activity were then compared to unravel the extent to which they contributed to sleep modulation by auditory information. In fact, it has been proposed that cardiac activity directly contributes to the way we perceive the world, but until now that evidence has been obtained in wakefulness. With these results, the ULiège researchers demonstrated that this also occurs during sleep, offering a new perspective on the essential role of bodily reactions beyond brain data for our understanding of sleep.
“Most sleep research focuses on the brain and rarely investigates body activity,” says Dr. Schmidt.
“However, we hypothesize that the brain and body are connected even when we cannot fully communicate, including sleep. Therefore, it is necessary to take into account both brain and body information to fully understand how we think and we react to our environment,” he explains. Dr. Demertzi.
“We freely share our methodology following the principles of Open Science in the hope that the tools that helped make this discovery will inspire other researchers to study the role the heart plays in other sleep functions,” says Dr. Koroma.
This work offers a more comprehensive approach to the modulation of sleep functions by sensory information. By observing cardiac responses to sounds, we can, for example, in the future study the body's role in how sounds influence the emotional processing of memories during sleep.