Summary: New research shows that the human body — and in particular the heart — continues to respond to sounds while we sleep, challenging the idea that sleep fully disconnects us from the environment. The study found that hearing relaxing words during sleep slows cardiac activity, a sign associated with deeper sleep, whereas neutral words do not produce the same effect.
These results highlight the importance of brain–heart interactions during sleep and suggest that both neural and bodily responses matter when sensory information influences sleep quality. The findings deepen our understanding of how auditory cues can modulate sleep and point to practical avenues for improving rest through targeted sound stimuli.
Key Facts:
- Heart rate responds to auditory stimuli during sleep: Listening to relaxing words while asleep can slow cardiac activity, indicating the body remains sensitive to external sounds.
- Brain–heart coupling in sleep: The study highlights significant interactions between brain signals and heart activity during sleep, showing the heart’s involvement in how auditory input affects sleep depth and quality.
- Open science and reproducibility: The research team has made their methods openly available, encouraging further studies into the heart’s role in sleep-related functions such as emotional memory processing.
Source: University of Liege
A team at GIGA — Center of Research Cyclotron, University of Liège, demonstrates that the sleeping body still senses and reacts to its surroundings, and that specific auditory cues can shape sleep depth.
Researchers at the University of Liège collaborated with the University of Fribourg to test whether the body is truly isolated from external stimuli during sleep. Prior work from this group examined brain activity (EEG) and found that exposure to relaxing words increased the duration of deep sleep and improved subjective sleep quality. Building on those findings, the current study examined cardiac responses to determine whether bodily signals also reflect this relaxation effect.
To explore body–brain coupling, the investigators analyzed heartbeat patterns measured by electrocardiography (ECG) while participants were exposed to different spoken words during sleep. They compared responses to relaxing words with responses to neutral control words. The key outcome was that relaxing words produced a slowing of cardiac activity consistent with deeper sleep, while neutral words did not trigger the same cardiac change.
The authors compared cardiac markers with previously recorded brain markers to tease apart how much each system — heart and brain — contributed to the observed modulation of sleep by sound. Although prior evidence for embodied perception has mainly come from studies in wakefulness, this study extends the concept to sleep: the heart, like the brain, participates in how auditory information is processed during rest.
By demonstrating cardiac slowing specifically after relaxing words, the research supports the view that sleep is not a state of complete isolation. Instead, both organ systems remain interconnected and responsive to meaningful external cues. This integrated perspective can refine how scientists study sleep and how clinicians approach interventions to improve sleep quality.
“Most sleep research focuses on the brain and rarely investigates bodily activity,” says Dr. Christina Schmidt. Dr. Athena Demertzi adds: “We propose that brain and body remain connected even when conscious communication is limited, such as during sleep. A full understanding of how we perceive and respond to our environment requires data from both systems.”
Lead researcher Dr. Matthieu Koroma emphasizes the team’s open-science stance: they have shared their methodology so other groups can replicate and build on the work. Making tools and procedures available supports reproducibility and encourages new investigations into how cardiac responses might influence other sleep-dependent processes, for example the way sounds affect emotional memory consolidation.
This study broadens the conceptual framework for how sensory information modulates sleep functions. By including cardiac responses alongside neural measures, researchers can form a more complete picture of the biological mechanisms that shape sleep depth and restorative processes. Future research may examine whether targeted auditory stimulation can be used as a noninvasive strategy to promote deeper sleep or to influence emotional processing during sleep.
About this sleep and auditory neuroscience research news
Author: Didier Moreau
Source: University of Liege
Contact: Didier Moreau – University of Liege
Image: The image is credited to Neuroscience News
Original Research: Closed access.
“Probing the embodiment of sleep functions: Insights from cardiac responses to word-induced relaxation during sleep” by Matthieu Koroma et al., Journal of Sleep Research