The researchers analyzed 196 nights of sleep using high-density electroencephalography with 256 electrodes.
Sleep is rarely reduced to just the number of hours spent in bed. The most significant factor often is the feeling of rest and alertness after waking up. This subjective sense of sleep depth can influence the entire day, but for a long time, scientists could not precisely explain what in the brain creates this sensation. A new study conducted at the IMT School for Advanced Studies (Italy) points to a possible answer: dreams. It turns out that vivid and emotionally charged dreams contribute to a feeling of deeper and restorative sleep, even if brain activity at night resembles wakefulness. These results contradict the old belief that deep sleep is only possible with a calm and minimally active brain.
Previously, deep sleep was associated with slow brain waves and low awareness: the deeper the sleep, the less active the brain. Dreams, however, typically occurred during the rapid eye movement (REM) phase, when the brain is almost as active as in a waking state, creating a paradox: how can sleep feel deep if the brain is 'awake'? The new study provides a more nuanced answer: not only the patterns of brain activity matter but also the mental experiences during sleep. The researchers analyzed 196 nights of sleep from 44 healthy adults, using high-density electroencephalography with 256 electrodes. Upon each awakening, participants described their feelings and rated the depth of sleep and sleepiness after waking. The study focused on the N2 stage of non-REM sleep, where the variety of dreams and subjective sleep sensations was particularly noticeable.
The results showed an unexpected finding: deep sleep was often associated with both a complete absence of conscious experiences and vivid, engaging dreams. In contrast, shallow sleep was more frequently marked by vague or fragmented impressions. Analysis of brain activity confirmed that high activity, usually associated with wakefulness, indicated shallow sleep, while low-frequency waves indicated deep sleep. However, this connection weakened if the person had dreams. In other words, brain activity resembling wakefulness did not make sleep shallow if the participant was immersed in a dream.
The researchers divided awakenings into three groups: those with vivid dream memories, those with a sense of dreaming without memories, and those without any experiences. Vivid dreams and a complete absence of conscious experiences were linked to a deeper feeling of sleep, while minimal experiences were associated with shallow sleep. The nature of the dreams also mattered: rich, emotional, and sensory dreams correlated with sleep depth, while abstract or 'thought-like' experiences correlated with shallow sleep. Even if participants could not recall the details of the dream, the feeling of having experienced an event influenced their perception of rest. Additionally, as the night progressed, the body's need for sleep decreased, and the brain became more active. Nevertheless, participants reported deeper sleep, which researchers attribute to an increased immersion in dreams. Vivid and engaging dreams helped maintain a sense of detachment from the external world—a key element of restorative sleep, even with heightened brain activity.
