Immersive NREM2 Dreaming Preserves Subjective Sleep Depth Against Declining Sleep Pressure

Behind the Veil of a Good Night’s Sleep

Good sleep depends not only on hours spent asleep, but also on the subjective feeling of deep, uninterrupted rest. In the present paper, we investigated the neural underpinnings and time-of-the-night variation of subjective sleep depth and its relationship with dreaming. To do so, 196 nights of sleep were recorded using high-density EEG and a serial awakening protocol. Upon each of 1,024 NREM2 sleep awakenings, participants were asked: “What was going through your mind just before you woke up?”, followed by questions related to subjective sleep depth, sleepiness, sensory content, and phenomenological dream features. To comprehensively categorise those reports, an extended classification of sleep-conscious experiences was used. This included: conscious experience with recall of content; conscious experience without recall of content (so-called “white dreams”); and no reported conscious experience, subdivided into conscious experience with a sense of presence (defined as a sense of being alive and/or perceiving the present moment and the passage of time), and complete unconsciousness.

Dream Immersion Shapes Perceived Sleep Depth

Our study showed that, as expected, deeper sleep was associated with reduced cortical activation, reflected in a lower high-to-low frequency power ratio. However, the presence and type of conscious experience modulated the relationship between brain activity and perceived sleep depth. This association was weaker during dreaming, i.e.,  wake-like cortical activation appears less disruptive to the perception of deep sleep when it occurs alongside dreaming. This suggests that immersive dreams may not be a simple by-product of sleep but rather play an active role in easing overnight fluctuations in brain activity, thereby sustaining the subjective feeling of deep sleep. Further, perceived sleep depth was lowest during experiences characterised by a mere sense of presence, and highest during either immersive dreaming or unconsciousness. 

Dreams: Overnight Guardians of Sleep

As expected, delta power – an objective marker of homeostatic sleep pressure – showed a gradual linear decline across the night. Intriguingly, perceived sleep depth increased as the night progressed, rising sharply in the second half, precisely when physiological sleep pressure was declining. Similarly, perceptual immersion in dream experiences increased significantly across the night, indicating that dreams became progressively richer and more immersive as the night unfolded.

These results show that standard objective measures of sleep depth tell only part of the good night’s sleep story. Beneath the veil, dreaming and subjective experience reveal a far richer one. Our results indicate that subjective sleep depth is not solely determined by reduced brain activity. Immersive dream experiences appear to maintain the subjective sense of deep sleep as homeostatic sleep pressure naturally declines across the night.

Tweak Dreams: Toward Deeper Sleep

If disrupted or less immersive dreaming contributes to poorer subjective sleep, a potential future direction is to focus on dream phenomenology. It could become a promising avenue for enhancing perceived sleep depth and, as a result, overall sleep satisfaction. Immersive dreaming, potentially achievable through dream engineering, may help buffer the arousing effects of cortical fluctuations supporting more restorative sleep.