Monitoring arousal levels among a warship crew: shift- and time of day-variations of saccadic velocity

Background: Due to the highly demanding nature of managing a warship, crewmembers are required to work in a rotating, very challenging shift schedule for very prolonged periods. The resulting short sleep duration and poor sleep hygiene can make difficult to maintain optimal arousal levels. Operational safety may be affected as a result of not optimal (i.e. reduced) arousal levels (i.e. sleepiness/fatigue). Here, we studied arousal variations of the crew of a replenishment oiler operated by the Spanish Navy before/after the shift and along the 24-hour cycle (morning, evening, and night shifts) for ten consecutive days. Methods: We conducted the study on board the Cantabria (A-15) during an international warfare training exercise (FLOTEX22). Twenty-six crewmembers (4 females, 34.67±7.62 years old) were assessed approximately every 4/6 hours. To assess oculomotor indices, we used the Fitness Impairment Tester 2000 (750 Hz, Pulse Medical Instruments Inc., US). The saccadic peak velocity was our main variable. In addition, we assessed intraocular pressure (Icare tonometer TA01, Tiolat Oy, Finland), objective/subjective sleep parameters, subjective levels of arousal and workload, as well as reaction times/errors with a psychomotor vigilance test (Fit-Alert, Miinsys-Optimal Solution S.A., Chile). Results: As expected, sleep time was always less than six hours, with great variability (and considerable sleep restrictions) depending on the shift. Although perceived workload was similar between shifts, fatigue (both subjective and physiological) was greater and performance worse at night shifts. Both saccadic peak velocity and intraocular pressure decreased after all shifts and were especially lower during the night shift. Conclusions: Our data confirm that saccadic peak velocity is a sensitive index of operator arousal levels. Overall, our findings support the viability of an objective, long-term, periodic monitoring of arousal levels in applied military settings. Real-time arousal assessment can support the monitoring of operator status for designing interventions to improve operational safety.