Perceptions of sleep depth don't correlate with sleep phase.
Sleep scientists generally categorize the various phases of human sleep as REM (rapid-eye movement – the period when dreams occur) and NREM (non-rapid-eye movement – the period typically described as “deep sleep”). Subjective perceptions of sleep and sleep quality or “depth” vary, however. Findings from a recent study suggest that some people’s perceptions of how deeply they sleep don’t align with objective measures of their sleep.
Objective measures of sleep quality are based on multiple physiological and neurological parameters that gauge the depth, quality, and duration of a person’s sleep. Typical measures include assessments of brain waves (via electroencephalogram, or EEG), heart rate, and respiration rate, among others.
The authors of the study recruited 20 healthy adults (average age, 38 years) who considered themselves good sleepers and 10 healthy adults of similar ages who had insomnia. While the participants slept, the authors measured the participants' brain waves via EEG, periodically waking them to obtain subjective assessments of their sleep depth and quality.
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The good sleepers reported that their sleep was lightest in the first two hours of NREM sleep (generally considered “deep sleep”) and deepest during REM sleep. Conversely, participants with insomnia reported feeling awake more often during the first two hours of NREM sleep and often reported having lighter REM sleep. Interestingly, the EEGs revealed that the participants with insomnia were asleep during periods when they reported being awake. During periods of subjective deep sleep, participants reported having dream-like mental activity.
These findings suggest that subjective perceptions about how deeply a person is sleeping don’t necessarily align with the phase of sleep they are in. Such findings could have relevance for future sleep studies, especially those aimed at improving sleep quality for people with insomnia.