The determinants of the subjective perception of sleep: insights from a high-density EEG study with serial awakenings

What determines the feeling of being asleep? Standard sleep recordings only incompletely reflect subjective aspects of sleep and some individuals with so-called sleep misperception frequently feel awake although sleep recordings indicate clear-cut sleep. Here we performed 787 awakenings in 20 good sleepers and 10 individuals with severe sleep misperception to interview them about their subjective sleep depth while they underwent high-density EEG sleep recordings (256-channels). Surprisingly, in good sleepers, sleep was subjectively lightest in the first two hours of Non-rapid eye movement (NREM) sleep, generally considered the 'deepest' sleep, and deepest in rapid eye movement (REM) sleep. Compared to good sleepers, sleep misperceptors felt more frequently awake during sleep, reported overall lighter REM sleep and had more thought-like conscious experiences. In both groups, subjective sleep depth positively correlated with dream-like features of conscious experiences. At the EEG level, spatially widespread high-frequency power was inversely related to subjective sleep depth in NREM sleep in both groups and in REM sleep in misperceptors. Taken together, these findings challenge the widely held notion that 'deep' (slow wave) sleep best accounts for feeling soundly asleep. Instead, they suggest that subjective sleep depth is inversely related to a neurophysiological process that predominates in NREM sleep early in the night, becomes quiescent in REM sleep and is reflected in high-frequency EEG-activity. In sleep misperceptors, this neurophysiological process is more active and spatially widespread, and abnormally persists into REM sleep. Thus, it is not the presence of 'sleep rhythms' but rather the absence of 'wake-like' EEG activity that predicts the feeling of being deeply asleep. These findings will help identify the neuromodulatory systems involved in subjective sleep depth and are therefore relevant for future studies aiming to improve subjective sleep quality.