High school start times after 8:30 am are associated with later wake times and longer time in bed among teens in a national urban cohort study.

OBJECTIVES: High school start times are a key contributor to insufficient sleep. This study investigated associations of high school start times with bedtime, wake time, and time in bed among urban teenagers.

DESIGN: Daily-diary study nested within the prospective Fragile Families and Child Wellbeing Study.

SETTING: Twenty US cities.

PARTICIPANTS: Four hundred thirteen teenagers who completed ≥1 daily diary report on a school day.

MEASUREMENTS: Participating teens were asked to complete daily diaries for 7 consecutive days. School-day daily diaries (3.8±1.6 entries per person) were used in analyses (N=1555 school days). High school start time, the main predictor, was categorized as 7:00-7:29 am (15%), 7:30-7:59 am (22%), 8:00-8:29 am (35%), and 8:30 am or later (28%). Multilevel modeling examined the associations of school start times with bedtime, wake time, and time in bed. Models adjusted for age, sex, race/ethnicity, household income, caregiver's education, and school type.

RESULTS: Teens with the earliest high school start times (7:00-7:29 am) obtained 46 minutes less time in bed on average compared with teens with high school start times at 8:30 am or later (P<.001). Teens exhibited a dose-response relationship between earlier school start times and shorter time in bed, primarily due to earlier wake times (P<.05). Start times after 8:30 am were associated with increased time in bed, extending morning sleep by 27-57 minutes (P<.05) when compared with teens with earlier school start times.

CONCLUSION: Later school start times are associated with later wake times in our large, diverse sample. Teens starting school at 8:30 am or later are the only group with an average time in bed permitting 8 hours of sleep, the minimum recommended by expert consensus for health and well-being.