Suppression of sleepiness and melatonin by bright light exposure during breaks in night work

Night work is non-optimal for performance and recuperation because of a lack of circadian influence that fully promote a night orientation. Our study assessed, in an industrial setting, the effects of bright light exposure (BL) on sleepiness, sleep and melatonin, during night work and during the following readaptation to day work. In a crossover design, 18 workers at a truck production plant were exposed to either BL (2500 lx) during breaks or normal light during four consecutive weeks. Twenty minute breaks were initiated by 67% of the workers between 03:00 and 04:00 hours. Sleep/wake patterns were assessed through actigraphs and ratings were given in a sleep/wake diary. Saliva melatonin was measured at 2-h intervals before, during and after night shift weeks. A significant interaction demonstrated a reduction of sleepiness in the BL condition particularly on the first two nights at 04:00 and 06:00 hours. Day sleep in the BL condition was significantly lengthened. Bright light administration significantly suppressed melatonin levels during night work and most strongly at 02:00 hours. Daytime melatonin during the readaptation after night work remained unaffected. The present findings demonstrate the feasibility and benefits of photic stimulation in industrial settings to increase adaptation to night work.