Arousals and macrostructure of sleep: importance of NREM stage 2 reconsidered

ABSTRACT Objectives: The sense of rest after sleep and its relation to various sleep parameters is still a debatable issue. The purpose of the present study was to analyse sleep fragmentation by scoring various arousals (microarousals (MA), vegetative (VA) and behavioural (BA) arousals) in all sleep stages and to evaluate their relation with subjective sleep quality without paying attention to the type of insomnia. Methods: The overnight sleep cycles of 60 subjects were analyzed according to their stage composition and arousals. Arousal indices (AI) were calculated for all types of arousals in all sleep stages and sleep cycles. The sleep quality was quantified using the Pittsburgh sleep quality index (PSQI). Results: AI differences between sleep cycles were not statistically significant. MAI value in total sleep time (TST) - 5.8 ± SD 4.1 - was the highest among all the three arousal types. Differences between AI in most sleep stages were statistically significant for all types of arousals. This suggests that human sleep development within a single sleep cycle is more important for the sleep quality than the changes between different sleep cycles. The highest AI scores for the three types of arousals were found in NREM stage 2. The strongest and significant correlation was between PSQI and MAI (r = 0.42; p = 0 .001). Conclusion: The density of microarousals is important for the subjective sleep quality. The highest values of MAI and other arousal types are found in NREM stage 2. The importance of this stage might be higher than thought before and especially in initial sleep cycles.