To date, studies investigating the consequences of shiftwork have predominantly focused on external (local) time. Here, we report the daily variation in cognitive performance in rotating shiftworkers under real-life conditions using the psychomotor vigilance test (PVT) and show that this function depends both on external and internal (biological) time. In addition to this high sensitivity of PVT performance to time-of-day, it has also been extensively applied in sleep deprivation protocols. We, therefore, also investigated the impact of shift-specific sleep duration and time awake on performance. In two separate field studies, 44 young workers (17 females, 27 males; age range 20-36 yrs) performed a PVT test every 2 h during each shift. We assessed chronotype by the MCTQ(Shift) (Munich ChronoType Questionnaire for shiftworkers). Daily sleep logs over the 4-wk study period allowed for the extraction of shift-specific sleep duration and time awake in a given shift, as well as average sleep duration ("sleep need"). Median reaction times (RTs) significantly varied across shifts, depending on both Local Time and Internal Time. Variability of reaction times around the 24 h mean (≈ ±5%) was best explained by a regression model comprising both factors, Local Time and Internal Time (p < .001). Short (15th percentile; RT(15%)) and long (85th percentile; RT(85%)) reaction times were differentially affected by Internal Time and Local Time. During night shifts, only median RT and RT(85%) were impaired by the duration of time workers had been awake (p < .01, consistent with the highest sleep pressure), but not RT(15%). Proportion of sleep before a test day (relative to sleep need) significantly affected median RT and RT(85%) during morning shifts (p < .01). RT(15%) was worst in the beginning of the morning shift, but improved to levels above average with increasing time awake (p < .05), whereas RT(85%) became worse (p < .05). Hierarchical mixed models confirmed the importance of chronotype and sleep duration on cognitive performance in shiftworkers, whereas the effect of time awake requires further research. Our finding that both Local Time and Internal Time, in conjunction with shift-specific sleep behavior, strongly influence performance extends predictions derived from laboratory studies.
