Morning–evening preference, weekday–weekend sleep-timing gap, and evening trajectories of subjective sleepiness

Individual differences in evening sleepiness may reflect both trait-like morning-evening preference and habitual variation in sleep timing. This study examined whether evening trajectories of subjective sleepiness differed by self-reported morning-evening preference and whether the weekday-weekend sleep-timing gap was associated with evening sleepiness independently of sleep duration. This secondary analysis used data from 212 adults (17–76 years) from the Subjective Sleepiness Dynamics Dataset. Participants completed the Karolinska (KSS) and Stanford (SSS) Sleepiness Scales at 20:00, 21:00, and 22:00, and reported habitual sleep schedules, morning-evening preference, and daytime sleepiness. Sleep duration, sleep midpoint, the sleep-timing gap, and the weekend-weekday duration difference were derived from reported schedules. Linear and linear mixed-effects models were used. Morning-evening preference was associated with habitual sleep timing, weekend sleep duration, and the sleep-timing gap. Sleepiness increased across the evening in all participants, but morning-preference participants showed an earlier increase than evening-preference participants. After adjustment for age, sex, preference, sleep duration, and daytime sleepiness, a larger sleep-timing gap was associated with higher overall SSS but not KSS scores, and was unrelated to the rate of evening sleepiness accumulation. These findings indicate that trait-like preference and state-like sleep timing provide complementary information about individual differences in evening sleepiness.