When we dream at night in the dark, the real world recedes and the imagination takes over. For a long time, researchers assumed that in these deeper stages of sleep, the body shut down, a kind of global dimmer switch powering off responsiveness to all sensory signals. But how cut off are we really?
To answer this question, a team of scientists from Switzerland recently hooked up 25 young healthy adults to EEG, tracking how they responded to clicks and tones wired in from the outside and, on the other hand, their own heartbeats when they were awake and when they were in different phases of REM sleep over two nights. The team was led by Marzia De Lucia, a neuroscientist and head of the Brain-Body and Consciousness Laboratory at the Lausanne University Hospital.
What De Lucia and her colleagues found is that the deeper the sleepers drifted into dreamland—particularly a state called phasic REM, the twitchy rapid-eye-movement phase associated with vivid dreaming—the more they responded to their own heartbeats, and the less they reacted to external noise. In some cases, the dreamers were even more attuned to their heartbeats than when fully awake.
Read more: “Dreaming Is Like Taking LSD”
Over the past decade or so, neuroscientists have become increasingly interested in how the body responds to its own internal states, such as heartbeat, breath, and gut signals, and how these fluctuations shape our emotions, perceptions, and experience of the self. This is called the science of interoception.
Apparently, the science of interoception can tell us something about what’s happening when we dream, too. It may involve a reallocation of attention, a turning inward toward one’s own body, the new findings suggest. This, the scientists say, could help explain why some people can sleep through loud noises but still wake up if they need to in an emergency or urgent situation.
It may also offer clues to why dreams in the deeper REM phases of sleep often feel more vivid, immersive, and emotionally intense: Because the brain is so closely tuned to its own sensations. The team suggests this hypothesis could be tested, combining measures of a dreamer’s responses to external and internal signals with dream reports.
The results are consistent with some evidence that bodily signals leak into our dream lives—a racing heart linked to a dream about being chased, for instance. And it connects to findings that people with nightmare disorder often have heightened heartbeat-evoked potentials, small brain waves measured by EEG that strike right after a person’s heartbeat.
Read more: “What Makes Us Lucid Dream?”
De Lucia and her colleagues published their results in Cell Biology, but note that they’re preliminary. The sample of dreamers was small, and the findings were only correlational, as well as limited to just one kind of external and internal signal each: sound and heartbeat.
To quantify the balance of how an individual participant responded to internal versus external signals, the team developed a new clinical tool that they say could have other uses: the audio-cardio index. In the future, it could serve as a measure of awareness for those in altered states of consciousness, particularly non-responsive states, they suggest.
So next time you dream of a heart-pumping chase up a mountain or down a dark alley, you may find that your racing heart isn’t just along for the ride, it’s in charge of the story. 
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Lead image: fran_kie / Adobe Stock
