Impact of advance light exposure on assembly-line workers' subjective work alertness and sleep quality

Underexposure to daylight in windowless factories puts assembly-line workers at risk of health problems. To investigate whether the advance light exposure in the early morning and during the lunch break benefits workers' productivity and night-time sleep, a within-subject experiment was conducted in a factory. Four experimental lighting interventions were provided twice a day before work (8:30-9:00 and 12:00-12:30) for consecutive five workdays, covering two light levels (1440 lx vs. 70 lx), each with two correlated colour temperatures (CCTs) (5300 K vs. 3300 K), plus an additional benchmark dark exposure (4 lx). Participants' subjective alertness and night-time sleep were measured daily, and the chronotype was monitored once a week. Results showed that a higher illuminance was correlated with increased subjective work alertness and higher sleep efficiency, while a lower CCT slightly improved alertness. The subjective work alertness and sleep efficiency under the two advance bright light exposures were higher than those under the dark exposure, and the responses of subjective alertness were more pronounced in the afternoon than those in the morning. Yet, the chronotype had no evident change in different lighting interventions. The present study indicated that the daily advance light exposure before work could have a delayed effect on participants' alertness and sleep quality.