a one hour later start time increases standardized test scores on both math and reading test by three percentile points. Since start times may be correlated with other determinants of test scores, I also estimate the effect using only variation in start times within schools over time and ﬁnd a two percentile point improvement. The effect of start times on academic performance is robust to different speciﬁcations and sources of variation. The magnitude of the effect is similar to the difference in test scores for one additional year of parental education.
The impact of later start times on test scores is persistent. Conditional on a high school ﬁxed effect, a one hour later start time in grade eight is associated with an increase in test scores in grade ten similar in magnitude to the increase in grade eight. … The impact of start times is greatest in grade eight (who are more likely to have begun puberty than those in the sixth or seventh grade
With a later school start time, kids will sleep later and better. This is crucial at the onset of puberty. The size of the effect is about 2% (on test scores). It carries forward to later years. It’s unclear how much this harm (which persists years later) is in merely lost learning (that could be made up) as opposed to lasting physical damage.
I was definitely permanently harmed by waking at 5:30am for over a year, to attend daily Mormon bible study (it’s supposed to run through all of high school, to protect us from the bad influences of less-conservative non-Mormon high school peers). I quit the church for other reasons, but in retrospect wish I had quit seminary immediately. I was 11-12 years old at the time and not close to fully grown. Sure, I would sometimes manage to sleep before 11pm, but others aren’t asleep by then, which makes it difficult, and even if they are, I often wanted to do my solitary things more than I wanted to sleep (not looking forward to the next morning, and wanting to prolong the sweet freedom of being awake).