It's claimed children could get better maths results if their lessons are in the morning.
The study, by an academic at Royal Holloway, University of London looked at academic achievement, class schedules and absence rates at a Bulgarian school over nine years.
They found the opposite is true for history classes - pupils do better in those lessons when they're taught in the afternoon.
The findings are due to be presented at the Royal Economics Society annual conference.
The findings back up psychology research which has found that it is better to perform repetitive, automatised tasks earlier in the day.
Tasks that involve making sense of something are better done later on.
