In a groundbreaking study conducted by researchers from the GIGA – Center of Research Cyclotron at the University of Liège, in collaboration with the University of Fribourg, it has been discovered that the human body, even while asleep, is not entirely disconnected from the external world. This study, published in the Journal of Sleep Research, reveals that our hearts respond to different types of words heard during sleep, with relaxing words causing a slow down in cardiac activity, reflecting a deeper state of sleep compared to neutral words.
Historically, the relationship between our body’s functions and cognitive processes, such as memory, emotion, and perception, has been extensively studied while we are awake. This concept, known as the embodiment of cognitive functions, suggests that our bodily states influence our cognitive processes. Previous research has highlighted the importance of cardiac activity in enhancing memory overnight, suggesting a link between our heart’s activity and sleep quality.
However, the mechanisms behind how cardiac activity during sleep influences cognitive functions remained largely unexplored. This gap in knowledge, coupled with the standard practice of overlooking the analysis of heart activity (ECG) in sleep studies, set the stage for the current research to delve deeper into how the heart’s responses during sleep can inform us about sleep quality and cognitive processing.
“Most of sleep research focuses on the brain and rarely investigates bodily activity”, said study author Christina 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,” added co-author Athena Demertzi.
The new study involved analyzing a dataset from previous research that had showed an increase in slow-wave sleep duration and subjective sleep quality when relaxing words were played during non-rapid eye movement (NREM) sleep. The original study, published in Sleep in 2021, involved 50 healthy, German-speaking subjects. These individuals were young, with an average age of 22.20 years.
Sleep was recorded and scored using standard methods to ensure accurate sleep stage identification. This included the use of EEG for brain activity, EOG for eye movements, and EMG for muscle activity, alongside ECG for capturing heart activity.
Participants were exposed to two conditions on separate nights: one where relaxing words were played and another where neutral (control) words were used. The timing of word presentation was strategically chosen to coincide with the peak of slow-wave sleep, a stage of deep sleep crucial for memory consolidation and recovery.
The researchers measured the heartbeat intervals before and after word presentation and compared these against a baseline of heartbeat intervals without word presentation.
During nights when relaxing words were played, participants’ heartbeats slowed down significantly, indicating a deeper level of sleep. This effect was not observed during the control nights. Furthermore, the study found that the heartbeats were modulated around the presentation of each word, with a more significant modulation for relaxing words than for neutral or reversed words.
The researchers also explored whether cardiac activity provided additional information about sleep depth beyond what could be gleaned from brain activity alone. By comparing cardiac and brain indices, they concluded that cardiac activity indeed holds unique information about sleep functions that brain markers alone cannot capture. This suggests a more complex interaction between our hearts and brains during sleep than previously understood.
However, the study is not without its limitations. The focus on cardiac activity during NREM sleep means that the findings may not necessarily apply to other sleep stages, such as rapid eye movement (REM) sleep. Additionally, the study’s methodology, relying on word presentation and heartbeat analysis, may not capture the full spectrum of sensory or cognitive processing that occurs during sleep. The researchers call for further studies to explore these areas and to extend the investigation of cardiac correlates of sleep functions.
“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,” remarked study author Matthieu Koroma.
The study, “Probing the embodiment of sleep functions: Insights from cardiac responses to word-induced relaxation during sleep,” was authored by Matthieu Koroma, Jonas Beck, Christina Schmidt, Björn Rasch, and Athena Demertzi.