A discovery by researchers in Switzerland reveals that the sleeping body also reacts to the external world during sleep, explaining how some information from the sensory environment can affect sleep quality.
A collaboration between University of Liège and University of Fribourg has investigated whether the body is truly disconnected from the external world during sleep.
To do so, they focused on how 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.
Matthieu Koroma (postdoctoral researcher), Christina Schmidt and Athena Demertzi (rsesearch) from the GIGA Cyclotron Research Center at ULiège teamed up with colleagues from University of Fribourg led 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 hypothesised 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 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 Dr Schmidt.
“We nevertheless hypothesise 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,” explains 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 advocates.
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.
Source: University of Liège
