Later school start times are linked to more sleep for teens, and the American Academy of Pediatrics (AAP) recommends that middle and high schools start no earlier than 8:30 a.m. Teens need about 8 to 10 hours of sleep, but only about 24 percent of US high school students got at least 8 hours on school nights in 2025 (CDC, CDC YRBS). Studies of schools that moved later show gains in sleep, and some show better attendance and grades. This guide explains the evidence, the policy picture and what families can do. It is for education only.
Check your teen’s own numbers with the adolescent sleep deficit calculator.
How Do Early School Start Times Affect Sleep?
Teen body clocks shift later at puberty. The AAP statement, as quoted by Sleep Review, says teens’ sleep-wake cycles move up to 2 hours later at the start of puberty (Sleep Review). Education Week reports that this keeps many teens from feeling sleepy until around 11 p.m. (Education Week). If class starts at 7:30 a.m., a teen who falls asleep at 11 p.m. gets far less than the 8 hours they need. For the body-clock basics, read circadian rhythm explained and teen sleep deprivation.
How Many Teens Get Enough Sleep?
Most do not. The CDC reports that about 1 in 4 students got at least 8 hours in 2023, and that the share getting 8 hours fell from 2013 to 2023 (CDC YRBS). The AASM summary adds that about 69 percent of US high schoolers sleep under 8 hours, and 85.6 percent of high schools start before 8:30 a.m. (AASM). In 2014, Education Week reported that about 40 percent of high schools started before 8 a.m. and only 15 percent started at 8:30 or later (Education Week). These figures come from different years and sources, so they show the direction, not one exact picture.
A Simple Example
Say a teen falls asleep at 11 p.m. and must wake at 6:30 a.m. That is 7.5 hours, which is below the CDC range of 8 to 10 hours. If school starts an hour later and the teen wakes at 7:30 a.m., that same bedtime gives 8.5 hours. Use the sleep calculator by age and gender to test your own schedule.
AAP School Start Time 8:30: What It Says
The AAP recommends that middle and high schools start no earlier than 8:30 a.m., so students can get 8.5 to 9.5 hours of sleep. It also tells schools to consider commute times and other local factors (Education Week). The CDC says teens need about 8.5 to 9.5 hours on school nights in its 2015 release, while its current sleep page lists 8 to 10 hours for ages 13 to 17 (CDC 2015, CDC). In 2015, the CDC found that fewer than 1 in 5 middle and high schools began at 8:30 a.m. or later, with an average start of 8:03 a.m. (CDC 2015). The AAP policy page itself was blocked when I checked, so the AAP details above come from news and trade reports.
Later School Start Times Study: What Happened In Seattle
Seattle moved two high schools from 7:50 a.m. to 8:45 a.m., which is 55 minutes later. Students wore activity monitors. The median gain was 34 minutes of sleep per school night, from 6 hours 50 minutes to 7 hours 24 minutes. Final grades were 4.5 percent higher (ScienceDaily). Median grades moved from 77.5 to 82.0 on a 100-point scale. At the school with more lower-income students, absences fell from 15.5 to 13.6 a year and first-period tardies from 6.2 to 4.3, and students did not stay up later after the change (Science News Explores). This was a small study of about 90 students per year at two schools, and each year’s sophomores were different students. So the results are linked to the change and do not prove it caused them.
What Other Studies Found
| Study | Main Finding |
|---|---|
| Review of 18 studies (AASM summary) | A start up to 60 minutes later added about 19 minutes of school-night sleep, and a delay of more than 60 minutes added about 53 minutes. Fewer crashes and less sleepiness were linked to later starts. Grade results were mixed (AASM). |
| CDC review of 38 reports | Most found more weeknight sleep. Many saw significant gains from delays of only about 30 minutes. Later starts were linked to better attendance, less tardiness, fewer dozing episodes and fewer teen driver crashes (Wheaton et al. 2016). |
| Minneapolis, 1997 | Seven high schools moved from 7:15 to 8:40 a.m. Students got about 5 extra hours of sleep a week and attendance improved in grades 9 to 11, but grades did not change significantly (Education Week 2001). |
The pattern is clear on sleep, fairly good on attendance and safety, and mixed on grades.
What The Evidence Cannot Tell Us
Be careful with any single result. Seattle was a small before-and-after study at two schools. Minneapolis found no significant grade change, and the AASM summary calls grade evidence mixed (Education Week 2001, AASM). The California results come from a working paper that has not been peer reviewed (NBER). The CDC also says later start times do not replace other sleep habits (CDC 2015). The safest reading is that later starts help teens sleep more, while effects on grades and mood need more research.
California School Start Time Law
California’s SB 328 added a rule to the state education code. High schools, including charters, may not start before 8:30 a.m., and middle schools may not start before 8:00 a.m. The deadline was July 1, 2022, or when the union contract in effect on January 1, 2020 ended, whichever came later. Rural school districts are exempt (FindLaw copy of Education Code 46148). FindLaw says its copy may not reflect the latest version, so check the state’s official site. A 2026 working paper from NBER, which has not been peer reviewed, evaluates the law. It reports a 13 percent rise in the share of students sleeping at least 8 hours and small gains in grade 8 math and English scores (NBER working paper).
Why Some Districts Push Back
Education Week reports that school officials have opposed later starts over after-school athletics and game schedules (Education Week). Florida passed a law requiring later high school starts, then repealed it in 2025. Most Florida high schools start around 7:45 a.m., and the state superintendents’ group cited supervision and student jobs after school. Districts that keep earlier starts must report their reasons to the state (WCJB). The CDC notes that start times are set by districts or schools, not by federal or state governments (CDC 2015).
What Families Can Do If School Starts Early
The CDC suggests the same bed and wake time every day, a quiet, relaxing and cool bedroom, turning off devices at least 30 minutes before bed, avoiding caffeine in the afternoon and evening and being active (CDC). The CDC also notes that later start times do not replace other sleep habits (CDC 2015). For more ideas, see sleep hygiene 101, social jet lag and how much sleep do you need. Younger children have different needs, so read children’s sleep by age.
Quick Takeaways
- The AAP recommends no school start before 8:30 a.m. for middle and high schools.
- Later starts are linked to more sleep, with mixed results for grades.
- Seattle’s gain was 34 minutes a night, from a small two-school study.
- California’s law has a rural exemption and a later deadline for union contracts.
FAQs About Teen Sleep And School Start Times
How Much Sleep Do Teens Need?
The CDC says teens aged 13 to 17 need 8 to 10 hours a night (CDC).
Do Later Start Times Raise Grades?
Not clearly. Seattle saw higher grades, Minneapolis saw no significant change, and the AASM summary calls the evidence mixed (ScienceDaily, Education Week 2001, AASM).
Do Teens Just Stay Up Later?
In Seattle, students did not stay up later after the change (Science News Explores). In Minneapolis, there was no sign they went to bed later either (Education Week 2001).
Are Later Starts Linked To Fewer Crashes?
Two reviews link later starts to fewer teen driver crashes. They show an association, not proof (AASM, Wheaton et al. 2016).
Which States Require Later Starts?
Sources
- Education Week (AAP recommendation, 2014)
- Sleep Review (AAP statement)
- CDC (2015 school start times release)
- CDC (about sleep)
- CDC YRBS (youth sleep)
- CDC YRBS (sleep behaviors)
- ScienceDaily (Seattle study)
- Science News Explores (Seattle study)
- AASM (review of later start times)
- Wheaton et al. 2016 (CDC review)
- Education Week (Minneapolis, 2001)
- FindLaw (California Education Code 46148)
- NBER working paper
- WCJB (Florida repeal)