The Relationship between Subjective and Objective Measures of Sleepiness

Abstract : The purpose of the present study was to examine the relationship among the Multiple Sleep Latency Test (MSLT), lapses during a tapping task, a visual analog scale (VAS) and the Stanford sleepiness scale (SSS). Subjects were 80 male adult nonsmokers (age 20.3 + or - 2.7 years). The MSLT, SSS, and the VAS were obtained at two-hour intervals beginning at 0700 h. On the MSLT, sleep latency was measured from lights out to first spindle, K-complex or rapid-eye- movement (REM) period. The tapping task (lapses) was administered each day at 0600 h, but became nonsignificant as the day progressed. Correlations of objective and subjective measures from scores summed over both days were not significant. The two objective measures were significantly correlated throughout the day and over days as were the subjective measures. This study reaffirms the importance of time of day in sleepiness, and suggests that subjective and objective measures cannot be used interchangeably and may measure different aspects of sleepiness.