Optimising lighting conditions to enhance seafarer adaptation to the '6-h on/6-h off' shift pattern: a balanced crossover study.

The '6-h on/6-h off' shift pattern could potentially disrupt the physiological rhythms and cognitive performance of seafarers, attributed to its shorter and more frequent shifts. Conversely, light exposure has been demonstrated to enhance cognitive abilities and synchronise physiological processes. Therefore, we studied the fatigue, cognition, sleep and rhythm of seafarers with different shifts to determine how light can benefit their performance. A total of 16 seafarers participated in a 2 × 2 crossover study, which involved two shift types (Morning-Evening and Day-Night) and two lighting conditions (static lighting and dynamic lighting). Sleepiness, cognition and fatigue were assessed every 2 h during '6-h on' period, using the Karolinska Sleepiness Scale, psychomotor vigilance task, critical flicker frequency and visual analogue scale for fatigue. Sleep was monitored during '6-h off' period, core body temperature was continuously tracked for rhythm throughout the shift protocol. For the Day-Night shift, the static mode with stable higher illuminance than dynamic lighting significantly reduced sleepiness (p = 0.01), objective fatigue (p = 0.001), subjective fatigue (fatigue level [p = 0.004] and visual fatigue [p = 0.001]) during the night period, while increasing sleep duration during the day (6:00 a.m. to 12:00 p.m.) and delaying the rhythm. For the Morning-Evening shift, dynamic lighting with lower illuminance significantly increased sleep duration during the night (12:00 a.m. to 6:00 a.m.) without causing a significant difference in performance. Overall, static lighting is more suitable for Day-Night shift seafarers due to lower sleepiness, fatigue and longer daytime sleep duration, while dynamic lighting is more suitable for Morning-Evening shift seafarers due to longer night-time sleep duration. Therefore, different lighting patterns should be adopted for seafarers during different shifts.