Districts that moved the high school bell later got 34 more minutes of sleep. Nothing else changed.

The most common objection to later school start times is that teenagers will simply stay up later, absorbing the change and gaining nothing. A four-year study of 61 districts that moved their high school start from an average of 7:35 a.m. to 8:30 a.m. found that this did not happen.

Measured with wrist actigraphy rather than self-report, students slept an average of 34 minutes longer on school nights. Bedtime moved later by about eight minutes — real, but far smaller than the gain.

Why bedtime barely moved

The explanation researchers offer is biological rather than behavioral. Adolescent circadian timing shifts later during puberty, and melatonin onset for most teenagers occurs around 10:45 p.m. regardless of what time they must wake. Teenagers were not choosing to go to bed late; most were already going to bed close to when they could fall asleep.

Moving the alarm therefore lengthened sleep almost directly, which is precisely what the physiology predicted and what the earlier, smaller studies had found.

The alarm clock was the intervention. Everything else was already as late as it was going to get.— The study’s lead author

What did and did not improve

Attendance improved by 1.9 percentage points and tardiness fell by roughly a third. Self-reported mood and daytime sleepiness improved on validated scales. Reported motor vehicle crashes among student drivers in the morning window fell, though the numbers are small enough that the authors treat that result cautiously.

Standardized test results did not move. The authors say this is unsurprising over four years for outcomes that reflect a decade of schooling, and warn against advocates on either side treating a null result as evidence for their position.

• Districts studied: 61 over four years, using actigraphy rather than self-report
• Start times: moved from about 7:35 a.m. to 8:30 a.m.
• Sleep: +34 minutes on school nights; bedtime later by about 8 minutes
• Outcomes: attendance +1.9 points; test scores unchanged

The practical obstacle in every district was transport. Most bus fleets run three staggered routes with the same vehicles, and moving the high school start compresses the elementary schedule. The districts that managed it either added buses, moved elementary earlier — which parents of younger children generally resisted — or negotiated a shared schedule with a neighboring district.