How much can we improve two-section watch bills? Sleepiness, Fatigue, and Psychomoter Vigilance Performance during naval operations in the 6/6 and 7-5-5-7 watch bills

A traditional two-section watch bill (6 hours on / 6 hours off, starting at midnight – “6/6”) and a two-section watch bill better adapted to the human circadian rhythm (watches of 7, 5, 5, and 7 hours duration, starting at 3 a.m. – “7-5-5-7”) were trialed on two vessels of the German Navy with 67 study participants in a crossover design. Questionnaires were used to assess subjective sleepiness (ESS, KSS) and subjective fatigue (FSS). The psychomotor vigilance task (PVT) was self-administered with wearable devices before and after each watch. Over the course of seven days at sea, PVT performance worsened significantly with the 6/6, but not with the 7-5-5-7 watch bill. In the 6/6 watch bill, number of participants with excessive sleepiness and high levels of fatigue was higher than in the 7-5-5-7 watch bill although not at a statistically significant level. Marked differences between watch bills in crew performance started to emerge after 6 days.