The human heart responds to external sounds while sleeping, findings of a new study reveal. Researchers found that listening to soothing words while sleeping can calm the heart and promote deeper sleep, in contrast to neutral words that have no impact on cardiac activity.

Scientists from the GIGA - Center of Research Cyclotron at ULiège, Belgium, working with counterparts from the University of Fribourg in Switzerland, discovered that the sleeping body responds to the external world. This challenges the notion that the body is entirely disconnected from the environment during sleep, and provides new insights into how sensory information can affect the quality of sleep.

Dr. Matthieu Koroma, Dr. Christina Schmidt, and Dr. Athena Demertzi from the GIGA Cyclotron Research Center at ULiège, along with a team from the University of Fribourg, conducted a previous study that found that relaxing words increased deep sleep duration and sleep quality, showing that we can positively influence sleep using meaningful words.

In the latest study featured in the Journal of Sleep Research, they analyzed their previous finding using electrocardiograms and investigated how heartbeat changes when we hear different words during sleep. They then found that the heart slows down its activity after the presentation of relaxing words, but not with control words.

Their findings revealed that exposure to relaxing words correlated with a decrease in cardiac activity, signifying deeper sleep. This was in contrast to neutral words, which did not elicit a comparable effect.

The researchers then compared markers of both cardiac and brain activity to understand their contributions to the modulation of sleep through auditory information. Before, it was thought that cardiac activity only affects how people perceive things when they are awake. However, this study showed that it also plays a role in shaping a person's perception during sleep.

"Most of sleep research focuses on the brain and rarely investigates bodily activity," said 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," explained 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 added.