Operational Efficiency and Time of Day

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Abstract
Forty-eight subjects performed a battery of six predominantly perceptual tasks that were relevant to bridge operations on a ship at sea. Sessions were run at six times over an extended waking day, between 0800 and midnight. For some of the tasks performance was noticeably deficient at certain times of day, the diurnal pattern corresponding approximately with that in activation and alertness. However, closer examination of the data reveals that although a genuine efficiency deficit may exist at particular times, a major part of the variation in performance can reasonably be attributed to a shift in the trade-off between speed and accuracy. Because the direction of the shift is towards faster but less careful performance as the day progresses, the shift may be due to accumulating fatigue rather than to time of day per se. Regardless of the primary cause, vulnerability may be increased at certain times due to a slump in operational efficiency.