When teens complain that school starts too early, they’re not wrong, according to new research.

This comes as school districts across the country—including in Colorado, California, Indiana, and Tennessee—consider starting school later.

The study, published last month in the peer-reviewed Journal of Human Resources, looks at districts in Florida and uses a novel approach: the fact that some areas in the state operate in the Central time zone while others use Eastern time. That means that if one district starts school at 8 a.m. Eastern and one right next door starts at 8 a.m. Central, students are actually heading to school at different times, relative to the sunrise—creating a natural experiment for the researchers to study how that affects student achievement.

Study authors Jennifer Heissel and Samuel Norris of Northwestern University followed students who move between schools in different time zones; they expected that students going from Eastern Time to Central Time will see their test scores improve because they get more sunlight prior to school.

In fact, that’s exactly what they found, particularly for older students. When an older student moved to a district that starts school later, their standardized test scores improved in the year they move and in later years. The effects are notable, but not huge: roughly equivalent to the impact of a substantial reduction in class size.

The research finds that is driven by the changes in ideal sleep patterns caused by entering puberty. That means girls are negatively impacted by late start times starting around age 11, and boys at age 13.