Impact of mobile display background Illuminance and spectrum on circadian rhythms, visual fatigue, and cognitive performance in nighttime environments

• 25-day experiment with physiological and psychological tests examined display effects. • Low CS displays improved visual and non-visual performance during nighttime. • CS metric was validated as an effective indicator of display circadian stimulation. • Circadian-friendly design of display should consider both CCT and CS parameters. • Print reading caused significantly less visual fatigue than smartphone displays. With the widespread adoption of smartphones, understanding their health impacts and mitigation strategies has become critical in night time lighting environments. This study investigated nighttime display effects on visual and non-visual performance through a 25-day experiment using a 2 × 2 repeated-measure design with Circadian Stimulus (CS: low vs. high) and Correlated Color Temperature (CCT: 4500 K vs. 7000 K) as variables. Participants' core body temperature, melatonin and cortisol concentrations, sleep quality, visual fatigue, and cognitive performance were systematically monitored. A Principal Component Analysis based model was developed to integrate various circadian indicators. Results demonstrated that display CS values exerted significant effects on circadian rhythms. Under low CS conditions, displays were associated with earlier timing of core body temperature minimum, earlier dim light melatonin onset, reduced cortisol concentrations, improved subjective sleep quality, diminished visual fatigue, and enhanced cognitive performance. Furthermore, among circadian quantification metrics including Equivalent Melanopic Lux, Melanopic Equivalent Daylight Illuminance, and Circadian Light, CS demonstrated the strongest correlation with circadian outcomes, validating its effectiveness as a display circadian stimulation indicator. Although CCT did not demonstrate statistically significant effects, displays with lower CCT values were subjectively perceived as more comfortable and conducive to superior sleep quality. These findings suggested that circadian-friendly display design should incorporate considerations of both CCT and CS parameters. Furthermore, the results indicated that participants reported significantly reduced visual fatigue when reading printed books compared to smartphone reading, with exposure duration identified as a critical determinant of visual fatigue intensity.