What does the strongest evidence show?
The most direct quantitative finding comes from a 2022 study of 6,571 Russian students in grades 6-11. After controlling for other factors, researchers found that students whose school started at 9:00 AM had significantly higher academic performance than those starting at 8:00 AM [1]. The effect was small but real — a statistical measure (B = 0.113) that translates to a noticeable, measurable improvement in grades. These students also slept more, had less 'social jetlag' (the mismatch between school-day and free-day sleep schedules), and were 30% less likely to show signs of food addiction [1].
A 2025 review paper summarizing evidence from the US, Europe, and Asia concluded that the primary advantages of later start times include increased cognitive functioning, better mental health, and improved academic performance [3]. The authors noted that despite this evidence, only a fraction of high schools globally have adopted later starts [3].
Is there a catch? Do the benefits last?
A 2024 study from South Korea provides a crucial reality check. Researchers tracked 1,133 adolescents from 7th through 12th grade as a policy delayed school start times by 30 to 90 minutes. Initially, students gained 19 minutes of sleep per night, and their smoking and drinking frequency dropped [2]. But after about a year, the sleep gain shrank to just 7 minutes — a difference that was no longer statistically significant. The reductions in smoking and drinking also faded [2]. This suggests that while later start times help, the benefits may erode as students adjust their bedtimes later, and that additional efforts may be needed to sustain positive impacts over time [2].
A 2026 qualitative study of 314 Swedish adolescents aged 13-15 who were about to shift from an 8:00 AM to a 9:00 AM start captured this ambivalence. Students expected better sleep and well-being, but also worried about longer school days and less free time [4]. Their views reflected both enthusiasm and apprehension, highlighting that the change is not a simple fix [4].
What about other benefits beyond grades?
Academic performance is not the only outcome. A 2023 US study of 2,426 high school students in Minnesota found that after schools delayed start times by 50-65 minutes, students showed modestly healthier weight-related behaviors: they were more likely to eat breakfast, have supper with family, get more physical activity, eat fast food less frequently, and eat vegetables daily [5]. Their body mass index (BMI) did not improve, but the behavioral shifts were in a positive direction [5]. This suggests that later start times can support broader health habits that may indirectly benefit academic performance over the long term.
Sources used in this answer
Later school start time is associated with better academic performance, sleep-wake rhythm characteristics, and eating behavior
Students starting school at 9:00 AM had significantly higher academic performance, slept more, had less social jetlag, and were 30% less likely to show food addiction compared to those starting at 8:00 AM (n=6,571).
Do later school start times improve adolescents' sleep and substance use? A quasi-experimental study
A later school start time policy in South Korea initially increased sleep by 19 minutes and reduced smoking/drinking, but these benefits shrank to a non-significant 7-minute sleep gain after one year (n=1,133).
Advantages of later school times for adolescents include increased cognitive functioning, mental health and academic performance: Is there a downside?
A 2025 review concluded that later school start times improve cognitive functioning, mental health, and academic performance, but noted that few high schools globally have adopted them.
Starting an hour later: Adolescents' concerns and positive expectations regarding introducing later school start times-A qualitative study.
Swedish adolescents (n=314) expected later start times to improve sleep and well-being but also worried about longer school days and less free time, showing ambivalence.
Impact of delaying high school start times on weight and related behaviors - the START study
After delaying start times by 50-65 minutes, US high school students (n=2,426) showed healthier weight-related behaviors (more breakfast, family meals, activity; less fast food) but no change in BMI.