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. 2022 Feb 18;12:2787. doi: 10.1038/s41598-022-06209-4

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© The Author(s) 2022

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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Comparison of sleep parameters between 8AM-days and ≥ 9AM-days in the flexible system. (A–F) Sleep parameters from the longitudinal cohort (n = 28) comparing 8AM and ≥ 9AM-days at t1 (light red) and t2 (dark red) intra-individually. (A) Average sleep onset, (B) offset, (C) duration, and (D) quality on 8AM versus ≥ 9AM-days in the flexible system across time points. Results of two-way repeated measures ANOVA with the within-subject factors school start (8AM/ ≥ 9AM) and time point (t1/t2) are reported above each graph. Brackets indicate statistically significant post-hoc comparisons. (E) Proportion of schooldays with alarm-driven waking. (F) Sleep gain on ≥ 9AM-days at t2 for each student. Depicted is the absolute difference in average sleep duration between 8AM and ≥ 9AM-days. Positive values mean longer sleep on ≥ 9AM-days. Dashed lines in violin plots show medians. All boxplots are Tukey boxplots. *p < 0.05, **p < 0.01, ***p < 0.001.