It is easy to lose heart over the prospects of improving American education, and in particular for narrowing achievement gaps between affluent and poorer students. Reforms are often expensive, politically fraught, or structurally complex (or all three). But once in a while, along comes a reform that is almost free, proven to be effective, and simple to implement. A perfect example: start high school later in the morning.

A raft of studies have shown that high school start times of 8:30 a.m. or later make for healthier, happier teenagers and higher test scores. There is a bigger positive impact for students from lower-income families, so later start times should narrow, even if just a little, the huge inequalities in educational outcomes. It is now well-established that most adolescents, as a result of changes in their body clock during puberty, will get their best sleep between 11 p.m. and 8 a.m. Right now, the average American high school starts class at 7:59 a.m. Many start even earlier.