August 10, 2026 | Source: Nautilus | by Kristen French
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.
