Does subjective sleepiness predict objective sleep propensity?

Research has shown variability in the correlations observed between subjective sleepiness and objective polysomnographic sleep latency. The present study evaluated whether eliciting subjective judgments of sleepiness after a 1-minute period of quiet with eyes closed would strengthen the relationship between subjective and objective measures.

Subjective judgments of sleepiness were collected following three 1-minute conditions (eyes-closed, eyes-open fixed gaze, and visual reaction time task) using the Stanford Sleepiness Scale and a visual analogue scale, prior to a measure of polysomnographic sleep latency. For each participant, subjective and objective measures were obtained a total of 12 times half-hourly from 20:00 to 01:30.

Sleep laboratory of the Flinders University.

Participants were 12 young adult good sleepers.

Within-subjects correlations between subjective and objective sleepiness across an evening period of increasing sleepiness were generally high (means approximately -0.63) and significant (P < 0.05). Contrary to expectation, there were no differences in correlations among the 3 conditions. Unexpectedly, when the correlations were calculated across subjects, correlations were noticeably weaker (means around -0.42), whereas the correlation means calculated across subjects, controlling for clock time, were close to 0.

In controlled laboratory conditions, high correlations between subjective and objective measures of sleepiness within subjects were found across a large range of sleepiness (20:00 to 01:30) for all conditions. Calculating correlations within subjects and across a range of the circadian variation in sleepiness contributed substantially to the strength of the relationship found. These results suggest that the variability of prior research findings may be due, at least partly, to the way in which correlations were derived.