The sleeping body reacts to the external world during sleep—with the heartbeat being impacted by certain stimuli—explaining how some information from the sensory environment can affect sleep quality, according to a discovery by researchers from the GIGA – Center of Research Cyclotron at University of Liège. 

Researchers at ULiège have collaborated with the University of Fribourg in Switzerland to investigate whether the body is truly disconnected from the external world during sleep. To do so, they focused on how the heartbeat changes when we hear different words during sleep. 

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, published in the Journal of Sleep Research, sheds new light on brain-heart interactions during sleep.

Researchers led a previous study analyzing brain data (electroencephalogram) showing that relaxing words increased deep sleep duration and sleep quality and 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 analyze cardiac activity 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 was so far 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 study co-author Christina Schmidt, PhD, in a release. 

Co-author Athena Demertzi, PhD, of the University of Liège, adds in a release, “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.” 

This work offers a more comprehensive approach about the modulation of sleep functions by sensory information. By looking into the cardiac responses to sounds, we may, for example, study in the future the role of the body in the way sounds influence emotional processing of memories during sleep.

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