Fig. 2.

Results for the study’s primary aim, showing the statistically significant effect of data collection week (A–C). Compared to baseline and the first data collection week of the lighting intervention (week 3), participants’ mean PSQI scores decreased (i.e., improved) significantly as the lighting intervention progressed, falling to close to the threshold for the absence of sleep disturbances (scores < 5, dashed line) by week 25 (A). Compared to baseline, participants’ mean sleep efficiency values increased significantly (i.e., improved) at data collection weeks 3, 17, and 25 (B). Sleep onset latency decreased (i.e., improved) between data collection weeks 3 and 9 but increased between weeks 9 and 25 (C). The values represent estimated means and the error bars indicate SEM. PSQI, Pittsburgh Sleep Quality Index; *p < 0.05; ***p < 0.001.