Associations between work start time with objective sleep and health-related outcomes: a 10-day field study distinguishing within-person and between-person effects

OBJECTIVES: This study examined the association between work start time (within-person and between-person effects) and work start time irregularity with objective sleep and health-related outcomes. METHODS: A 10-day field study was conducted with 143 employees. Daily work start times, blood pressure (BP), subjective state, brief psychomotor vigilance test, and actigraphic sleep variables were assessed. Work start time was decomposed into within- and between-person components using person-mean centering. Work start time irregularities were quantified using the interquartile range of work start times during the observation period. Linear mixed models were used. RESULTS: Earlier-than-usual work start times (within-person level) were significantly associated with shorter actigraphic total sleep time (TST; β = 6.50), earlier lights-off time (β = 0.10), bed-out times (β = 0.24), higher systolic (β = -0.80) and diastolic (β = -0.74) BP after waking. In contrast, earlier average work start times (between-person level) were significantly associated with earlier lights-off time (β = 0.63) and bed-out times (β = 0.76), but not TST and BP. Greater work start-time irregularity was significantly associated with higher systolic and diastolic BP before bedtime (β = 2.38 and β = 1.92) and after waking (β = 2.77 and β = 2.04). CONCLUSIONS: Earlier work start times were associated with sleep and health-related outcomes, although the patterns differed between within- and between-person levels. Work start time irregularity was associated with elevated pre-bedtime and post-waking BP; however, associations with sleep and other outcomes were limited. These findings underscore the importance of distinguishing within- and between-person effects.