The Sleep Regularity Index: A New Way to Evaluate Shiftwork Schedules.

Sailors adhere to a variety of demanding shiftwork schedules (known as 'watchbills') which pose a challenge for sleep and wellbeing at sea. Previous research aimed at identifying viable watchbills based on how they protect sailors' sleep has largely relied on sleep duration. Findings have highlighted insufficient sleep during watchstanding but have mixed results when comparing across watchbills. Sleep regularity is another important dimension of sleep that has been rarely assessed during watchstanding. This study aimed to investigate sleep regularity assessment as a tool for evaluating watchbills, using a recently introduced metric-the Sleep Regularity Index (SRI). Two hundred eighteen sailors from different watchbills were assessed for approximately 1 week of an underway period. The median SRI score was significantly lower for the 5-h on/15-h off watchbill compared to all other watchbills, despite no statistically significant differences in daily sleep duration. The median SRI score was similar between fixed watchbills. The large measurable differences in SRI scores between watchbills, particularly when sleep duration was similar, demonstrated the value of sleep regularity assessment as a tool for evaluating watchbills.