Research sheds new light on brain-heart interactions during sleep

A discovery by researchers from the GIGA – Center of Research Cyclotron at the University of Liège reveals that the sleeping body also reacts to the outside world during sleep, explaining how certain information from the sensory environment can affect the quality of sleep.

Researchers at ULiège collaborated with the University of Friborg in Switzerland to investigate whether the body is actually disconnected from the outside world during sleep. To do this, they focused on how heart rate changes when we hear different words during sleep. They found that relaxing words slowed heart activity as a reflection of deeper sleep and compared to neutral words that did not have such a slowing effect. This discovery is presented in 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 ULiège collaborated with colleagues from the University of Friborg, led a previous study analyzing brain data (electroencephalogram) showing that relaxing words increased the duration of deep sleep and sleep quality, showing that we can positively influence sleep by using meaningful words. Until then, the authors assumed that the brain also remains able to interpret sensory information in a way that makes our body more relaxed after hearing soothing words during sleep. In this new study, the authors had the opportunity to analyze heart activity (electrocardiogram) to test this hypothesis and found that the heart slowed its activity only after the presentation of relaxation words, but not control.

Indices of both heart and brain activity were then compared to distinguish how much auditory information contributed to sleep regulation. Cardiac activity has indeed been suggested to contribute directly to how we perceive the world, but such evidence has so far been received in the waking state. 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 in our understanding of sleep.

Most sleep research focuses on the brain and rarely investigates physical activity.”

Dr. Christina Schmidt, GIGA Cyclotron Research Center at ULiège

“However, we assume that the brain and body are connected even when we cannot fully communicate, including during sleep. Brain and body information must be taken into account to fully understand how we think and react to our environment,” he explains. Dr. Demertzis.

“We freely shared our methodology following the principles of Open Science, hoping that the tools that helped make this discovery will inspire other researchers to study the role the heart plays in other functions of sleep,” says Dr. Koroma.

This work offers a more comprehensive approach on the modulation of sleep functions by sensory information. By examining cardiac responses to sounds, we can, for example, in the future study the body’s role in how sounds affect the emotional processing of memories during sleep.