Table 3. Adjusted Difference-in-Differences Analysis of Changes in Sleep Variability and Continuity Among High School Students Before and After a 50- and 65-Minute Delay in School Start Timesa.
| Sleep variability and continuity | Delayed-start schools | Comparison schools | Difference-in-differences analysis | |||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Baseline (n = 122) | Follow-up 1 (n = 141) | Follow-up 2 (n = 90) | Baseline (n = 160) | Follow-up 1 (n = 162) | Follow-up 2 (n = 94) | Baseline to follow-up 1 | P Value | Baseline to follow-up 2 | P Value | |
| Weekday-weekend midpoint difference, h:minb | 1:51 (1:34 to 2:08) | 1:30 (1:17 to 1:43) | 1:34 (1:20 to 1:49) | 1:57 (1:46 to 2:08) | 2:13 (2:00 to 2:27) | 1:54 (1:35 to 2:13) | −37 (−58 to −17) min | <.001 | −14 (−37 to 9) min | .22 |
| School night sleep efficiency, %b | 85.9 (84.8 to 87.0) | 85.7 (84.8 to 86.6) | 84.6 (83.7 to 85.6) | 85.4 (84.6 to 86.2) | 84.8 (83.9 to 85.7) | 85.5 (84.3 to 86.7) | 0.4 (−0.9 to 1.7) | .53 | −1.4 (−2.8 to 0.0) | .054 |
| School night SFIb,c | 30.1 (28.2 to 32.1) | 31.1 (29.6 to 32.7) | 32.8 (31.1 to 34.5) | 30.0 (28.7 to 31.3) | 28.8 (27.3 to 30.3) | 27.9 (25.7 to 30.0) | 2.2 (0.0 to 4.5) | .053 | 4.8 (2.2 to 7.5) | <.001 |
| School night sleep onset latency, minb,d | 8 (6 to 10) | 11 (9 to 12) | 12 (11 to 14) | 9 (8 to 10) | 12 (11 to 13) | 12 (10 to 14) | 0 (−2 to 2) | .91 | 1 (−1 to 4) | .22 |
| Net school night sleep duration (subtracting out wake-ups after sleep onset), h:mine | 6:40 (6:28 to 6:52) | 7:04 (6:54 to 7:14) | 6:59 (6:48 to 7:11) | 6:57 (6:47 to 7:07) | 6:45 (6:35 to 6:54) | 6:46 (6:33 to 6:58) | 37 (23 to 50) min | <.001 | 31 (16 to 46) min | <.001 |
Abbreviation: SFI, sleep fragmentation index.
Data are from the START study, spring 2016 through spring 2018. Baseline occurred in spring 2016 (9th grade); follow-up 1, spring 2017 (10th grade); and follow-up 2, spring 2018 (11th grade). Unless otherwise indicated, data are expressed as mean (95% CI).
Linear mixed-effects models adjusted for demographic factors and length of daylight, with school fixed effect in place of school random effect.
Calculated as the proportion of the sleep period characterized by movement.
Indicates number of minutes from “lights off” to sleep onset.
Linear mixed-effects models adjusted for biological sex, socioeconomic status (free or reduced lunch eligibility and parent educational attainment), race and ethnicity, and length of daylight at actigraphy, school- and student-level random effects.