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Trend for sleep in last week and air quality index (AQI) concentration among the freshman cohort. Survey time 1: 2013–2014 cohort (9–15 Dec); 2: 2013–2014 cohort (5–11 May); 3: 2014–2015 cohort (6–12 Oct); 4: 2014–2015 cohort (24 Feb–2 Mar); 5: 2014–2015 cohort (4–10 May); 6: 2015–2016 cohort (14–20 Sep); 7: 2015–2016 cohort (2–8 May); 8: 2016–2017 cohort (21–27 Nov); 9: 2016–2017 cohort (15–21 May); 10: 2017–2018 cohort (13–19 Nov); 11: 2017–2018 cohort (30 Apr–6 May).
