Do 8am Classes Lower Your Grades? What Three Natural Experiments Found

Assignment to a 7:30 a.m. section at Purdue University lowered course grades by about 0.05 GPA points, the gap between a 3.00 and a 2.95. Early classes do cost grades on average, but the studies disagree sharply about how much, and the size of that disagreement is the most useful thing they have to teach.

Why do the Air Force Academy and Purdue results look so far apart?

At the US Air Force Academy, Scott Carrell, Teny Maghakian and James West found that a start 50 minutes later raised achievement by roughly what a one-standard-deviation improvement in teacher quality would (“A’s from Zzzz’s? The Causal Effect of School Start Time on the Academic Achievement of Adolescents,” American Economic Journal: Economic Policy, 2011, volume 3, issue 3, pages 62 to 81). At Purdue, a 7:30 a.m. assignment moved a course grade by five hundredths of a point. Put next to each other, one looks like a large intervention and the other like rounding error.

Some of the gap probably comes from what each study measured. Carrell’s team used two changes to the Academy’s daily schedule, layered on random placement of first-semester students into courses and instructors, and looked at achievement across the day. Students with the earliest start did worse in the classes that followed the first period, which fits the idea that the cost comes from being tired and not from anything about the first-period course. Yim measured grades in a single course, in the section a student was assigned. That is an interpretation of why the numbers differ, and neither paper tests it directly.

The two settings also differ in who was studied. Cadets live under military scheduling and got in through a competitive admissions process. Purdue’s freshmen are ordinary students at a large research university. The Purdue result also shows that a small grade effect in one course need not mean a small effect on everything downstream.

What did Purdue find beyond the 0.05?

Anthony Yim, an economist now at Brigham Young University, studied administrative records covering freshman course enrollments after Purdue began assigning class times in a way that effectively randomized them across sections. His paper, “How Early Morning Classes Change Academic Trajectories: Evidence From a Natural Experiment,” appeared in the Journal of Policy Analysis and Management (working-paper version 2024, published 2026).

The larger results were about what came next. Students in early sections were roughly 4 percentage points less likely to take another STEM course (about 14 percent fewer), and about 3.1 percentage points less likely to major in the same college (around 29 percent fewer). Yim also ran a survey in an introductory course, including students assigned to the 7:30 section, to look at why.

A small grade effect can grow into a larger trajectory effect. A student who gets a B-minus in the 7:30 intro course instead of a B may hesitate before the next course in the sequence. That is a guess about mechanism, and the paper’s data cannot say what any single student was thinking.

Which students end up in 8am sections, and why does it matter?

Ask a registrar and the answer is not random. Students with jobs, athletes with afternoon practice, and students who register last get the early sections, while students confident about their study habits steer clear of them. A researcher who simply compares grades across early and late sections is measuring the hour and the kind of student who picks it at the same time, and the two cannot be separated.

Economists call the fix a natural experiment: find a place where an institution assigned the schedule, or changed it for everyone at once, and compare the groups that result. The Academy and Purdue are two such places. The third study below is a different animal.

What did a Singapore university’s Wi-Fi logs show?

A team at Duke-NUS Medical School and collaborators, writing in Nature Human Behaviour in 2023 (“Early morning university classes are associated with impaired sleep and academic performance”), pooled several kinds of digital traces from tens of thousands of students at a university in Singapore: Wi-Fi connection logs, learning-management-system logins, and grade records.

Three results stood out:

• Lecture attendance was about ten percentage points lower for classes at 8:00 than for later starts.
• Students slept about an hour less on nights before early classes, because they woke earlier than they otherwise would have.
• The number of days per week a student had morning classes was negatively correlated with grade point average.

Since students in this dataset did choose some of their classes, the grade result is an association and not a cause. Its value is that it fills in a step the two randomized studies could only imply: the lost sleep and the missed lectures.

Do early bells hurt grades for younger students too?

Two school-start studies add context, though they come from a different age group.

In Wake County, North Carolina, Finley Edwards of Colby College used data on every middle school student in the district from 1999 to 2006. He compared the same schools before and after their start times moved (Economics of Education Review, volume 31, issue 6, pages 970 to 983, 2012) and found that an hour later raised math scores by about two percentile points.

In Seattle, the school district moved its secondary school start from 7:50 a.m. to 8:45 a.m., and a University of Washington team led by Horacio de la Iglesia compared students before and after. The results, in Science Advances in December 2018 (Dunster et al.), showed a median gain of 34 minutes of sleep a night, and students’ final grades in the class were 4.5 percent higher.

Look at how much sleep those students gained. The bell moved 55 minutes and their sleep moved 34, so roughly 60 percent of the schedule change turned into sleep. The rest went to a later bedtime or a slower start to the night, which the data cannot split apart.

So how much do 8am classes cost, in one paragraph?

On average, 8am classes lower grades by a small amount, and the strongest evidence comes from two randomized studies. At the US Air Force Academy (Carrell, Maghakian and West, 2011), a start 50 minutes later raised achievement by about as much as a one-standard-deviation improvement in teacher quality. At Purdue University (Yim, 2026), assignment to a 7:30 a.m. section lowered course grades by about 0.05 GPA points and reduced later STEM course-taking. A Singapore dataset covering tens of thousands of students pointed to the likely path: about an hour less sleep before early classes and attendance about ten percentage points lower at 8:00.

What can these studies not tell you?

The Air Force Academy is not a typical campus, and the title of Carrell’s paper says “adolescents.” The sleep habits of a cadet may not resemble those of a sophomore living off campus with a night job. The Seattle and Wake County results describe middle and high school students, and biological sleep timing shifts across those ages, so their numbers cannot be dropped onto 20-year-olds. Purdue’s students are a better match, but Yim’s result covers freshman enrollments at one university, in the sections where times were effectively randomized.

The averages also hide who is affected. An average penalty of 0.05 GPA points can come from a small loss for everyone or a large loss for a few students whose sleep timing sits furthest from the schedule. None of the studies followed a single student who moved from an 8am schedule to an afternoon one and tracked the difference in her own record.

Adaptation is the other open question, and it matters for anyone weighing a whole semester of 8am starts against a single early course. A student who takes early classes all semester might get used to them. The data here are mostly single courses or single semesters, so whether adjustment happens over four years is unknown.

What does this mean if you are building a schedule?

The evidence supports a modest reading. If two sections of a course fit your life equally well, the later one is likely the better bet, particularly for the first course in a sequence you want to continue. That is a statement about odds and not about individuals, and the penalty Yim measured was tiny in grade terms. If an early section is the only one that fits around a job or a required lab, the grade cost is small enough that it should not outweigh the practical reasons for taking it.

For students who cannot avoid an early class, the sleep numbers suggest where the problem lies. The Singapore students slept about an hour less on early-class nights, and the Seattle students recovered only 34 of their 55 minutes. The distance between your body clock and the class schedule, which researchers call social jet lag, probably decides how much you pay. Chronotype varies, and so does how much sleep a given person needs, so the average is a starting point and not a forecast for anyone in particular.

Loose ends

Do early classes hurt strong students as much as weaker ones?

Probably not equally. In the Wake County middle school data, an hour’s later start raised math scores by about two percentile points, which Edwards put at roughly fourteen percent of the black-white test score gap, and the gain was larger for students at the low end of the score distribution. Neither the Purdue nor the Air Force Academy papers reported that kind of split in the results summarized here.

What else changed in Seattle besides sleep and grades?

Absences and tardiness fell. The study followed sophomore biology students at two high schools, with the pre-change measurements taken in spring 2016. Median nightly sleep rose from 6 hours 50 minutes to 7 hours 24 minutes after the bell moved from 7:50 to 8:45 a.m.

Do the school-start-time studies apply to college students?

Only loosely. The Seattle and Wake County studies covered high school and middle school students, whose body clocks run later than those of younger children and, on average, adults. College freshmen are close in age to the older end of that group, but no one has run the same policy change on a college-wide schedule.

How large was the Singapore dataset?

It combined three sources of different sizes: Wi-Fi connection logs for 23,391 students, learning-management-system logins for 39,458 students, and grade records for 33,818 students. Because the groups differ, the sleep, attendance and grade findings each rest on a different slice of the university.

Sources: Carrell, Maghakian and West, American Economic Journal: Economic Policy 3(3), 2011, aeaweb.org; Yim, Journal of Policy Analysis and Management, 2026, doi:10.1002/pam.70088; Edwards, Economics of Education Review 31(6), 2012, sciencedirect.com; Early morning university classes study, Nature Human Behaviour, 2023, nature.com.