Objective components of individual differences in subjective sleep quality.

The relation between subjective and objective (polysomnography) is of obvious interest in understanding sleep. The aim of the present study was to examine the item structure of the Karolinska Sleep Diary (KSD), and the covariation between KSD and sleep-stage variables, with cross-sectional (inter-individual) data obtained in natural conditions. Thirty-seven subjects had a polysomnography prior to a work day. After rising they completed the Karolinska Sleep Diary. A factor analysis of the diary yielded two factors: (1) a sleep-quality index (SQI) related to the initiation and maintenance of sleep, and (2) a second factor related to difficulties waking-up and to whether sleep was recuperative and sufficient. The strongest predictors (multiple regression) of the sleep quality index was slow-wave sleep (SWS) and sleep efficiency [and variables related to sleep continuity total sleep time (TST) and the amount of stage 0]. The obtained results were in agreement with previous laboratory findings. It was concluded that sleep quality seems to be a matter of depth of sleep and sleep continuity.