The more vivid the dream, the deeper the sleep feels
Dreams can influence how restful we perceive our sleep to be. The results call into question the view that the feeling of deep sleep is exclusively due to slow brain activity.
The key to a restful night's sleep lies not only in its duration, but also in the subjective impression of having slept deeply. A team from the IMT School for Advanced Studies in Lucca, Italy, has now investigated the role that dreams play in this. They found that the more vivid the dreams are, the deeper the sleep feels in retrospect, as the experts report in «PLOS Biology».
For the experiment, Adriana Michalak and her colleagues invited 44 healthy adults to the sleep laboratory four times each and recorded their brain waves using EEG. The team woke the participants up several times a night and asked them about their experiences immediately before waking up as well as their perceived depth of sleep and sleepiness. They did not wake them at random, but always during the so-called non-REM sleep phase 2 (N2). At around 50 per cent of the total sleep time, this stable light sleep accounts for the largest proportion of the sleep cycle.
The more vivid the dream, the deeper the sleep feels.
Though dreams occur primarily in REM sleep, they also occur in N2 (and deep sleep) and are associated with faster brain waves similar to those experienced during wakefulness. Normally, slow brain activity is associated with the feeling of deep sleep. This was also shown in this study. What the experts also found, however, was surprising: Those who remembered vivid and intense dreams after waking felt that their own sleep was particularly deep. In contrast, rather vague dream sequences were associated with the impression of very light sleep. «The quality of the experience, especially how vivid it is, seems to be decisive», explains study leader Giulio Bernardi in a press release. «The more immersive the dream, the deeper the sleep feels.»
Not just a question of brain activity
The results challenge the commonly held view that the feeling of deep sleep is exclusively due to slow brain activity. Instead, intense dreaming appears to help maintain the subjective experience of deep, restorative sleep - even when sleep pressure decreases during the night. They could also explain why some people feel that they sleep poorly even though the usual objective sleep parameters appear normal.
However, the authors point out that the repeated waking may have influenced the dream content as well as the perceived depth of sleep, which makes a comparison with normal nights difficult. Furthermore, as there are still no technical possibilities to read out dreams directly, it is difficult to derive causal relationships.
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