0779 Circadian Entrainment in 12h Night Shift Workers: The Environmental Lights Matter?

Abstract

Abstract Introduction Shift workers frequently suffer sleep disturbance in relation with the atypical work schedules. In particular, night shift workers are exposed to inevitable sunlight or bright indoor lights during the morning hours that may disturb their daytime sleep. We aimed to find out the optimal environmental lights to facilitate circadian entrainment of 12h-shift workers by assessing melatonin profiles in the morning and daytime sleep quality. Methods We enrolled 12h-shift female nurses working at one hospital (n=10, mean age 29.4±3.5 years). The schedules of participants are identical such as day-day-night-night shifts and four consecutive off days. Participants admitted to the laboratory at 9:00 following 2nd night shift schedule. Saliva melatonin was taken six times every 30 minute from 10:00 to 12:30 under two different lighting conditions with organic light emitting diodes (OLED) or light emitting diodes (LED) with 150 lux of light intensity. Dim light condition (10 lux) was conducted as controls. Three sessions were randomly ordered with 8-10 days intervals. Participants were allowed to sleep after 12:30 with light off and woke up ad libitum. Results Melatonin concentration had decreased gradually from 10.6±11.4 to 5.0±8.2 pg/ml. Among three different lighting conditions, there were no statistical differences in salivary melatonin and sleep parameters recorded by polysomnography. Circadian entrainment in night shift workers was defined as that salivary melatonin concentration had maintained above 5 pg/ml at the time to bed. 20% of sessions (6/30) were classified as circadian entrainment (CE) and 80% (24/30) were as non-entrainment (NE). Mean melatonin concentration was 22.1±11.2 in CE and 3.9±4.1 pg/ml in NE (p<.001). CE showed significantly shorter sleep latency (0.5±0.3 vs. 1.5±1.4 min, p=.025) and wakefulness after sleep onset (13.6±6.3 vs. 27.9±16.8%, p=.015), and higher sleep efficiency (94.6±2.6 vs. 88.7±6.3%, p=.011) than NE. The number of each lighting condition was not different between CE and NE (p=.847). Conclusion This is a preliminary study with small number of participants. We found that environmental lights including dim light did not affect daytime sleep of 12h shift workers. Instead, daytime sleep quality was influenced by circadian entrainment with higher melatonin concentration in the morning. Support