The feeling of having had "a good night's sleep" lies not only in how much we slept, but also in the subjective impression of having slept deeply and without interruption. But what constitutes the neural base of this perception is not very well understood.
Now, a new study by researchers at the IMT School for Advanced Studies Lucca, appeared in Plos Biology, suggests that the dreams, especially the most vivid and immersive ones, rather than disrupting sleep, could help it feel deeper and restoring.
For years, we thought deep sleep meant a "switched off" brain: slow brain waves, little activity, no awareness. In this view, the deeper the sleep, the less active the brain. On the other hand, dreaming has been instead associated with Rapid Eye Movement (REM) sleep and it is acknowledged to reflect partial "awakenings" of the brain. Yet, surprisingly, this stage marked by intense dreaming and wake-like brain activity is also commonly experienced as a relatively deep sleep.
To investigate this paradox, researchers analyzed 196 overnight recordings from 44 healthy adults who slept in a laboratory while their brain activity was measured with high-density electroencephalography (EEG). The data were collected in a larger study supported by a European Research Council (ERC) Starting Grant investigating how various kinds of sensory stimulations impact on the subjective experience of sleep.
Dreaming changes how brain activity relates to sleep depth
Across four laboratory nights per participant, the researchers collected more than 1,000 awakenings with corresponding reports, creating one of the largest datasets linking brain activity, dream experience, and subjective sleep perception. For the experiment, participants were awakened repeatedly from non-REM sleep, a stage characterized by broad variability in both subjective sleep depth and dreaming, and asked to report their mental experiences just before awakening, and to rate perceived sleep depth and subjective sleepiness.
The results revealed that the deepest subjective sleep was reported not only when participants had no conscious experience, but also after vivid and immersive dreams. By contrast, minimal or fragmentary experiences, such as a vague sense of presence without clear dream content, were associated with the shallowest perceived sleep. "In other words, not all mental activity during sleep feels the same: the quality of the experience, especially how immersive it is, appears to be crucial" explains Giulio Bernardi, professor in neuroscience at the IMT School and senior author of the study. "This suggests that dreaming may reshape how brain activity is interpreted by the sleeper: the more immersive the dream, the deeper the sleep feels".
The authors also found another striking element: although physiological markers of sleep pressure steadily decreased across the night, participants paradoxically reported feeling that their sleep was becoming deeper. This subjective deepening closely tracked a rise in the immersiveness of dreams, suggesting that dream experiences may help sustain the feeling of deep sleep even as the biological drive for sleep wanes. Indeed, immersive dreams may help maintain our sense of disconnection from the external world, a defining feature of restorative sleep, even as parts of the brain become more active.
Dreams as "guardians of sleep"
"Understanding how dreams contribute to the feeling of deep sleep opens new perspectives on sleep health and mental well-being," says Bernardi. "If dreams help sustain the feeling of deep sleep, then alterations in dreaming could partly explain why some people feel they sleep poorly even when standard objective sleep indices appear normal. Rather than being merely a by-product of sleep, immersive dreams may help buffer fluctuations in brain activity and sustain the subjective experience of being deeply asleep". This idea echoes a long-standing hypothesis in sleep research - and even in classical psychoanalysis - that dreams may act as "guardians of sleep."
