"Objective
This study aimed to investigate sleep and circadian phase in the relationships between neurobehavioral performance and the number of consecutive shifts worked.
Methods
Thirty-four shift workers [20 men, mean age 31.8 (SD 10.9) years] worked 2–7 consecutive night shifts immediately prior to a laboratory-based, simulated night shift. For 7 days prior, participants worked their usual shift sequence, and sleep was assessed with logs and actigraphy. Participants completed a 10-minute auditory psychomotor vigilance task (PVT) at the start (~21:00 hours) and end (~07:00 hours) of the simulated night shift. Mean reaction times (RT), number of lapses and RT distribution was compared between those who worked 2–3 consecutive night shifts versus those who worked 4–7 shifts.
Results
Following 4–7 shifts, night shift workers had significantly longer mean RT at the start and end of shift, compared to those who worked 2–3 shifts. The slowest and fastest 10% RT were significantly slower at the start, but not end, of shift among participants who worked 4–7 nights. Those working 4–7 nights also demonstrated a broader RT distribution at the start and end of shift and had significantly slower RT based on cumulative distribution analysis (5th, 25th, 50th, 75th percentiles at the start of shift; 75th percentile at the end of shift). No group differences in sleep parameters were found for 7 days and 24 hours prior to the simulated night shift.
Conclusion
A greater number of consecutive night shifts has a negative impact on neurobehavioral performance, likely due to cognitive slowing."

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