Quick self-assessment of night sleep quality with reliable and valid instruments is often required in various fundamental and applied studies in the field of sleep medicine and chronomedicine. Individual differences in sleep quality are usually assessed with a special scale, whereas such a scale is absent in most of questionnaires for evaluation of individual differences in the domain of chronobiology. One of two exceptions is the 6-scale Sleep-Wake Pattern Assessment Questionnaire. It contains the 12-item scale S designed to self-assess nighttime sleepability. The purpose of the present report was to examine external validity of this scale by searching for correlates and predictors of S score. One hundred and sixty participants of sleep deprivation experiments self-reported their sleep latency, total sleep duration, nap frequency, and times for going to bed and awakening for one week prior to the experiment. Their objective and subjective sleepiness after sleepless night was measures in the course of experiment. Moreover, sleep latency and total sleep duration were measured by means of polysomnography in studies with 35 female participants. Other objective measurements included percentages of sleep stages, awakenings after sleep onset, and sleep efficiency. Additionally, sleep latencies were detected during 9 day- and nighttime 20-min napping attempts of 15 from 35 participants. It was found that significant correlates and predictors of S score include subjective and objective measures of sleep latency, percentage of slow wave sleep, subjectively and objectively assessed sleepiness, and self-reported total sleep time. These results provided solid evidence for external validity of scale S.